leetcode笔记(六)740. Delete and Earn
- 题目描述
Given an array nums
of integers, you can perform operations on the array.
In each operation, you pick any nums[i]
and delete it to earn nums[i]
points. After, you must delete every element equal to nums[i] - 1
or nums[i] + 1
.
You start with 0 points. Return the maximum number of points you can earn by applying such operations.
Example 1:
Input: nums = [3, 4, 2]
Output: 6
Explanation:
Delete 4 to earn 4 points, consequently 3 is also deleted.
Then, delete 2 to earn 2 points. 6 total points are earned.
Example 2:
Input: nums = [2, 2, 3, 3, 3, 4]
Output: 9
Explanation:
Delete 3 to earn 3 points, deleting both 2's and the 4.
Then, delete 3 again to earn 3 points, and 3 again to earn 3 points.
9 total points are earned.
Note:
- The length of
nums
is at most20000
. - Each element
nums[i]
is an integer in the range[1, 10000]
.
- 解题思路
看题目,可以意识到是动态规划类型的题目,但不知道怎么写迭代式子,就是相不清楚状态。所以,一开始心虚的按照自己的贪婪算法实现了下,结果就是代码极多,但结果不能对~
无奈,开始查阅资料,找到了一篇比较靠谱的博客。借鉴他的思路,自己努力写了下动态规划的实现。思路的关键在于:
- 取出一个数,其收益为 数的频数 × 数的值。按照规则,取出一个,必然取出该值的所有数。
- 两个状态,取出当前数的最大收益(maxFetch),不取当前数的最大收益(maxNoFetch)。
- 初始状态:
- maxFetch = 0, maxNoFetch = 0;
- 当前状态与上一状态的关系
- 不取当前数,则为上一状态的最大值(max(prevMaxFetch, prevMaxNoFetch))。
- 取出当前数,若数和上一状态的数关联(+/- 1),则为prevMaxNoFetch + 取出数的收益。否则,为max(prevMaxFetch, prevMaxNoFetch) + 取出数的收益。
- 示例代码
class Solution {
public: int deleteAndEarn(vector<int>& nums) {
map<int, int> freqs;
int size = nums.size();
for(int i = ; i < size; i++)
{
int curr = nums[i];
// remember frequences for curr
if(freqs.find(curr) == freqs.end())
{
freqs[curr] = ;
}
else
{
freqs[curr] += ;
} } int maxFetch = , maxNoFetch = ;
int prevMaxFetch = , prevMaxNoFetch = ;
map<int, int>::iterator prevChoice;
map<int, int>::iterator currChoice;
// calculate maximum according to previous status
for(currChoice = freqs.begin(); currChoice != freqs.end(); ++currChoice)
{
// initiate
if(currChoice == freqs.begin())
{
// get this number
maxFetch = currChoice->first * currChoice->second;
// do not get this number
maxNoFetch = ;
}
// transferring
else
{
prevMaxFetch = maxFetch;
prevMaxNoFetch = maxNoFetch;
// do not get the number
maxNoFetch = max(prevMaxFetch, prevMaxNoFetch);
// get this number
if(currChoice->first == prevChoice -> first + || currChoice->first == prevChoice -> first - ) {
// related -> must not fetch previous node
maxFetch = prevMaxNoFetch + currChoice->first * currChoice->second;
}
else
{ // non related
maxFetch = maxNoFetch + currChoice->first * currChoice->second;
}
} prevChoice = currChoice;
} return max(maxFetch, maxNoFetch);
}
};
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