算法面试题:一个List,要求删除里面的男生,不用Linq和Lamda,求各种解,并说明优缺点!

解题思路

这是群里某位小伙伴去面试碰到的面试题,从题目本身来看,面试官应该是要考察面试者对泛型 List 的理解程度,也算是对基础的理解。这里面还是有很多需要考察的知识点,没关系,我们走一步看一步。

首先,题目没有限定数量,我们知道,只谈毒性不谈剂量的都是耍流氓,那我们就假定要处理的数据有 10000 条好了。

数据准备好了

static readonly string text = "1000100010101001101101011110111111100000100010001101010101001101111010100001010101100101011011101011111010101000111100110101001001000010100000010111010110011010111100000101110011011100100010110101100010111010100010110000001011110110000011110001101001000000111100110101101111111000011100010110010000000101111010111011100000101111000011101101110100111010101001010000001101000100111011111101010100011011000001001010100100001001011001100011001010111110110100101101101010010100010101011100110101110011011101001001001110101101011010101001000011001111010101110101010001111010110001011100010001000011000101000011010011011000100011111110111010001111111000100011110000100010010101000111110111111101111111000000001011010101000100100101010100000111101111010011011011100000001001000110011100110110001110100100110010011100000110111010010011100011001100010001011100010010001011100110101110000001000010001111101010110000111111110101110100111101110000111000110011110101001110010001010100110110010100011110110111010010011011100011010111011010100110011010001110011101000100010100011110001000101010100000010111011010101100001111011000001110110010011111011000010001101100001101100001010010000001001101110101110001100000100100100110101110110110101001111011011010111000100110011011111001010001100001011000110000100011000010010101011010100111001000110001011101010100010101111101100101001110100111000011110101111111001011100001001001100110011001010001111000110000101111010101010011101010111000000001001111001111101010101010101101100101010111001011100000010001011001000001110101000110111000011110101010110101101001001101001110000010000000010011111110101001111111000100100010001100101111011111000110100000000000110010011101010010011100000000111111000001011100000101100001100010101111010110000100011101101101111011100010011111100100100001001001011001000000010100110101110101101100001000011110001100010101011000100000000011101001101101100101010011000101100100001010100011100111110011100110110011100011100011110001101100000001111011100111110011111100001100001111101101111101110001111011010100011000101000111110110111011000011110010110010010100011100101111000101110010110100000011010000101110100001101001001110010110101101011110110001111101101000000011011010001111000010110000011110100101011011100100111001101001101110011101010001001001010010111100010111111111001101010000011000000000101111101011110111001001000010100000010100000110101001101100001111001011100000101010000010000111101100111011101001111000011110111011100011101110000101111000111011101010100000110011000010001101101110000111010111100001011110001101100001000110101010010001010000000001110111011110001110101000000101110010100110010110101011100000100010011100001001010100110100100101111010111100000101010100100101001010110000101110110010110100101101111000010100001010011111111010011101001001011001000101000010000000100111001011010110000010001110010011100010111111100101001010100111000100011001111100101100011100000101100110010101100110111100000001111111011000100010100110111011011001000010110100000001111011010001010101110010111101000010110001010110011001010100101010001110100110100111101010011011110011101011111100101011010101101010010000100110111000011011110110000010110110000001010101011110001110001101100110111011010001001010011110100011111110110110000000111100111100111001110111011011010010100101110001111001110001100110011111001001000110011101111111100111011001001010011101100010000111010111000111101010101100010100101000001000111000010001010110110100010100011010110111110111111101001101000010111000111111011001100001111110011111100000110100110000011010110100111001001100111001010011011011000111100001001111100100101101101010111101001101001000101000100101000010110010100001111001010010111101001100011100010111001110100111111101110011100000011110110100001100001011110110100011010010001100000011111110110001001101001010010010000001000111000110101111000011110111011111010100010011011100111010101101010101000000111001111101001001010011011001101111000000111111111111101001011011000001100110111100011100110010111001111001011100001100001100000001010100011010101111101111011010001100010000001101110100111111100111011010110101000110111101001011010100001101001010011110011111101010100010110111001000001110101001111000001100100110100100010110001111001100011111010000101011101001100001101110111111001111100010110101010110011011100101110100001001110011101101011011010100011100110101100000101001111101101000011110000110011011110100111110000001000010100011000110100000111100011101101000000111111101010000111101100100110001011111100011101010011001000001001000111011111101000110111011000110100110000010100001000100101010110011010011110100100000000001000101011000001010011011011110101110001001101110001100110101011001110010111000101001111101011011011000100100011100110100011000111111110100101000001101000000111101111101100000111011110111001101010011010011011010001100010010100101001000111111110001100100001001111010000101010101111011000111111011101110100110010100000001010000101011101011001110111001100010100101111010111100100010111011110100010001001010100110011110110010110101010010011111010011111111101111100110110000101110000110101100111110111011000001101110100110011010010110001111110001110100011011101110000100101110110101010100010110111100010100110100101110001001011011010100001001110100100110101110110100111010010011001001010000101010100100100100101110101001000100110110100011010101101010101011011001001000001101101000000100001011010110010010111010111101001011100110000100010110000100001100110011110101101000101011100010111111000100011111111111111001101011111010100100100010001000111111010110001111000110011111001001100100000010010101100000101011000001101111001010110000001100111111001111101101111101001011110101011111101100010100100100100110001110011010101111110100000011000100111101110010001001101010001101100110010101111011011001001010010101111100100001111010101001010000001001010110101111100111111101001101011101001100101000001000000001001110100010011001110110010011010000011100011011110100010111111011001000001101100111111110110111111111011110110100100100011010000111110001101011000100100110010011101111001010011100100101001001110001000101010000001100000111101000111011001000101110100010101000110100101100001100011111010001010100111001111111111000100111001110011010101101001010111110000110111010000100011000000000101010000101011100010100111101110110010100111001111000101110011111001100101011101011110101011010010111000101100000001100000001001011011111110101000100101110111001001111100000010110110010011001111101110101010000011110001010100001010010111101010011101100010011110111001111110111110000110001011110101001100110101000000010111101011100000010110101010101001110011011001100101001011111101010110101010010111010101110101010001111001111110010111001000100001010101011100111010100001000100111110101111000010000001111111101001010110000101111101110000011011110001011010101100110111001000010110001011001100101110100101100111001000000000111111010110010111011000100100001110110100010001010101110100110111100111010011000001011101001010111011101101000001000111110000011011011010100011111110101100111111101001001001111001100001001110011100001011111001000001011000010100100110101001111100111001000000001000111001000000000010011000111000111111010100000001101010001001010001011011101100010110111010000110001110000110100001001011101001001011111000111010010101101111000100101010001101110110010011011110000101111111111110111101010100101101000011010001101101100101011100011011101100110101011100110100010101011000010110110111101001001101111101011010111010101100010001001010011111100010100111101100001100100111111011110101111111111000100000001001010110011001101001000001111000100110110001110010001011110010110001010110001011110010010000000100100001111111001110011111111000111010000001100000000011001110100000011101110000100100110110101111110101100101100001000001001000100001011110100000011010100111001110100111110101010011010010011101000111011000111001111011111100010110100010101001010100110100000000111111100011010110001111010000100001110000010100111101101001101100110111010010101110101010110011000011000111010100011100000001111110100110010000111000010001010011111011101000110000011100100111000100000111011010101000110010111100010111010011101001000111001101000001110100010110101100101111111000001110010100011111101001110110101100011101100100001111001001111110000001011000100001100110001001101000111111001100001010111010001001011011111110011111110001000110111101011011101010000001100010000010010011010110010100010000101010100000101010100010011010101010100110111000001110100100100011100101000000010001011010001011111110111110000110110111000010010001000000101000000100101110110111011010010111000000101110001001000010010111110111010110000000111101010111111011000001010010010001101010000111101000111011100111011001010011001001101000111111111110000110100110101110110010010100011100001000010001001011111110001111111001100001110110011111110100000011110010011110111111111011000001111101100110111011101000100111001000000101100101101001100111110100111010010110101010111101001000010111010011000010000110111100011011110000010011111001010100100000100000100110000100111000110001010100101001001011010000011010110101100100101011010000001010000011110100101001101000100100001000101100100110101111001001010110101010011111100000111110011101011000001101011000010000001110010101010100000111000100011111100101000010011010110111110011100010111101100101111101101110000100001100010101011101100001101000011110100100101100110000011010010100000011111000111111001010010011110110011010100100010111110110100001101110101111110001000000001000111111011011010000000010010111101011110011100000111000000100111001010010011110010010100100100010101100001001011011101110011110001000011011001001011100111101000100101110110110011000101010010111000111111011111100000010011010101110000101000001110011001001011111100011111010100111100010110001111010100010100001110000101101100000111100101000011010010010111010111001011101011011000111";

看到上面的这堆 0110 是不是有点懵逼?

没关系,我刚看到的时候也是一样的,用下面的代码把他转换成数组就好

List<int> students = new List<int>();
for (int i = 0; i < text.Length; i++)
{
students.Add(text[i] - 48);
}

你看,事情往往就是这么简单、有趣,一个 for 循环就完美解决了我们的麻烦。

方案一:暴力解题法

看到这个题目,我们常常第一时间就是想到循环,遍历找出这帮真男人(1)就好了不是?

来吧,下面就是一段暴力 for 循环

List<int> newStudents = new List<int>();
for (int i = 0; i < students.Count; i++)
{
if (students[i] == 1)
newStudents.Add(students[i]);
}

你看,这种以暴制暴的方式往往就是这么枯燥、无趣。

知识点来了!!!

由于我们声明了一个 newStudents 的泛型对象,这个对象在初始化的时候,默认大小是多少?正确答案是 4 。是的,你没看错,咱 .NETCore 就是这么扣,就是任性,就给你 4 个长度的列表。

那咱们就要问了,这 4 个是怎么装下这么多男人(1)的呢?

动态扩容

答案是:当我们调用 List.Add() 的时候,如果当前列表长度小于容量,即 Length < Capacity,那么数据就正常的装入列表中,如果即将装入的数据长度 + 现有的列表长度 > 容量,即 Length > Capacity,那么 List 底层就会进行自动扩容。

每次扩容多少合适呢?翻倍!

没错,就是这么简单有效,咱直接翻倍了,不装了,不算了,就是这么简单有效,咱就翻倍。

看 List 的底层扩容的实现

private void EnsureCapacity(int min) {
if (_items.Length < min) {
int newCapacity = _items.Length == 0? _defaultCapacity : _items.Length * 2;
// Allow the list to grow to maximum possible capacity (~2G elements) before encountering overflow.
// Note that this check works even when _items.Length overflowed thanks to the (uint) cast
if ((uint)newCapacity > Array.MaxArrayLength) newCapacity = Array.MaxArrayLength;
if (newCapacity < min) newCapacity = min;
Capacity = newCapacity;
}
}

重新分配内存扩容

看上面的代码,当调用 Capacity = newCapacity;进行扩容的时候,内存将进行重新分配,具体做法就是在属性 Capacity 内部,使用 newCapacity 声明一个新的数组,然后把源数组中的内容复制到新数组中。

public int Capacity {
get {
Contract.Ensures(Contract.Result<int>() >= 0);
return _items.Length;
}
set {
if (value < _size) {
ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.value, ExceptionResource.ArgumentOutOfRange_SmallCapacity);
}
Contract.EndContractBlock(); if (value != _items.Length) {
if (value > 0) {
T[] newItems = new T[value];
if (_size > 0) {
Array.Copy(_items, 0, newItems, 0, _size);
}
_items = newItems;
}
else {
_items = _emptyArray;
}
}
}
}

所以,由此推断,我们一开始的暴力迭代方案将会在运行时发生多少次内存分配和 GC 呢,我们算一下,4/8/16/32/64/128/256/512/1024,算了,我算不过来了,反正就是很多很多!

暴力修正方案

从上面的知识点可以得知,我们应该一开始就应该声明一个合适长度的 List。

但是,我们又没有办法知道有多少男人(1)呀,那怎么办,这种时候就要逆向思维,声明一个足够大的数组,你源数据不是有 10000 条吗,那我就假定 10000 个都是男人(1),声明一个 10000 人的列表好了。

List<int> newStudents = new List<int>(students.Count);
for (int i = 0; i < students.Count; i++)
{
if (students[i] == 1)
newStudents.Add(students[i]);
}

完美!

方案二:算法实现

既然是算法题,如果只考察内存和GC,那未免显得我们学识粗浅,下面我们对数据进行排序,然后再截取数组实现解题。

冒泡排序

我们这里使用经典的冒泡排序法,当然你可以二分或者其它,随意发挥好了。

int total = 0;
for (int i = 0; i < students.Count; i++)
{
if (i == 1)
total++;
for (int j = 0; j < i; j++)
{
if (students[j] < students[i])
{
var stu = students[i];
students[i] = students[j];
students[j] = stu;
total++;
break;
}
}
}

上面的代码实现了对数据进行排序,排序的同时不要浪费机会,借机获取了男人(1)的数量,为下面的节省内存分配做准备,既然学了就要应用是吧。

截取数组

int[] newStudents = new int[total];
students.CopyTo(0, newStudents, 0, newStudents.Length);

截取数组的代码非常简单,首先利用排序得到的数量声明合适的列表,然后利用 List.CopyTo() ,方法即可得到男人(1)。

这里的 students.CopyTo() 底层实现顺便告诉大家,使用的是 Array.CopyTo()。是的,你没看错,咱

这是 List 的底层的实现

public void CopyTo(int index, T[] array, int arrayIndex, int count) {
if (_size - index < count) {
ThrowHelper.ThrowArgumentException(ExceptionResource.Argument_InvalidOffLen);
}
Contract.EndContractBlock(); // Delegate rest of error checking to Array.Copy.
Array.Copy(_items, index, array, arrayIndex, count);
}

本质上都是调用 Array 的方法。

方案三:修剪法

经过两轮磋商,相信这个时候面试官想必对你已经是刮目相看,别着急,下面还有一个方案,利用迭代列表,去修剪列表中的女人(0),剩下的自然都是男人(1)。

for (int i = 0; i < students.Count; i++)
{
if (students[i] == 0)
{
students.RemoveAt(i);
}
}
  • 注意了,这是一段错误的代码!

  • 注意了,这是一段错误的代码!

  • 注意了,这是一段错误的代码!

上面的修剪代码虽然看似执行了修剪女人(0)的过程,实际上,我们要了解的是,在修剪的过程中, students.Count 会随着修剪的执行过程而同时会发生改变,最终结果就是,修剪长度被缩短,导致部分女人(1)没有得到修剪检查。

看 List.RemoveAt() 的内部实现。

public void RemoveAt(int index) {
if ((uint)index >= (uint)_size) {
ThrowHelper.ThrowArgumentOutOfRangeException();
}
Contract.EndContractBlock();
_size--;
if (index < _size) {
Array.Copy(_items, index + 1, _items, index, _size - index);
}
_items[_size] = default(T);
_version++;
}

List.Count 属性的返回值正式上文中的 _size;所以,你现在明白这个方案为什么是个错误了吧。

方案三的修正版本

那么我们就没有办法使用修剪方案了吗?

不是的,请年轻人先把40米的大刀收一收,我们来修正一下上面的修剪方案。

for (int i = 0; i < students.Count; i++)
{
if (students[i] == 0)
{
students.RemoveAt(i);
i--;
}
}

具体做法就是在执行修剪操作后,将当前下标 i 和 students.Count 进行同步,一个小小的改动,我们实现了修剪。

性能问题

通过上面的三种方案,我们学习了 List 的内部执行机制,了解了内存分配和GC的回收过程,那么这三种方案的性能怎么样呢?

是时候上图了。

总结

好了,今天的面试就到这里结束。

通过上面的性能对比,我们可以看到排序方案的耗时最高,耗时高达 166ms ,而其它两个方案的耗时都为 0,当然,从内存占用来看,排序方案肯定是要优秀一些的了,但是在这个求快的时代,内存这个可怜的家伙很多时候都会被我们安排在优化的第二梯队。

而从综合考虑,修剪方案较为可行(仅针对本题)。

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