[抄题]:

There is a new alien language which uses the latin alphabet. However, the order among letters are unknown to you. You receive a list of non-empty words from the dictionary, where words are sorted lexicographically by the rules of this new language. Derive the order of letters in this language.

Example 1:

Input:
[
"wrt",
"wrf",
"er",
"ett",
"rftt"
] Output: "wertf"

Example 2:

Input:
[
"z",
"x"
] Output: "zx"

Example 3:

Input:
[
"z",
"x",
"z"
] Output: ""  Explanation: The order is invalid, so return "".

[暴力解法]:

时间分析:

空间分析:

[优化后]:

时间分析:

空间分析:

[奇葩输出条件]:

[奇葩corner case]:

相同的c2,只需要存一次(没有就新存 有就不存),反正如果存过 就必须退出了(返回一组即可 不用重复加)

"["za","zb","ca","cb"]" How is this test case handled. 
It should give out an empty string as the order can not be decided from the words given. but instead it returns "azbc". 回答:we can only now z-> c and a-> b
so 'azbc' is right but right result is not limited to this only one.
you can test 'zcab', 'abzc' are will all right as it is topological sort

结果字符串长度不等于度数(不是不等于单词数)

["z","z"]的度数 = 字符串长度1
正常,应该返回"z"而不是""

[思维问题]:

忘了拓扑排序用BFS怎么写了

[英文数据结构或算法,为什么不用别的数据结构或算法]:

DFS写拓扑排序似乎都很麻烦

存点到点的对应关系,用map(其中的字符必须存成包装类Character,但是循环的时候可以写char)

        //存每个点的入度
Map<char, Integer> degree = new HashMap<>();
//存c1到c2...的对应关系
Map<char, Set<char>> map = new HashMap<>();

[一句话思路]:

[输入量]:空: 正常情况:特大:特小:程序里处理到的特殊情况:异常情况(不合法不合理的输入):

[画图]:

[一刷]:

  1. 出现index[i + 1]时,需要提前备注把上线变为n - 1

[二刷]:

  1. BFS的时候别忘了吧把取出的c1添加到结果中去. 而且必须现有c1的key才能扩展。

[三刷]:

[四刷]:

[五刷]:

[五分钟肉眼debug的结果]:

[总结]:

c1 c2都要先判断一下有没有,再存

[复杂度]:Time complexity: O(n^2 单词数*字母数) Space complexity: O(n)

[算法思想:递归/分治/贪心]:

[关键模板化代码]:

BFS的存储和扩展是两个独立的步骤,扩展时必须先判断key是否存在,再做扩展

[其他解法]:

[Follow Up]:

[LC给出的题目变变变]:

[代码风格] :

[是否头一次写此类driver funcion的代码] :

class Solution {
public String alienOrder(String[] words) {
//ini:HashMap<Char, Integer> degree, store all chars into hashmap, String res
//存每个点的入度
Map<Character, Integer> degree = new HashMap<>();
//存c1到c2...的对应关系
Map<Character, Set<Character>> map = new HashMap<>();
String res = "";
for (String word : words) {
for (char c : word.toCharArray())
degree.put(c, 0);
} //compare and store, n - 1
for (int i = 0; i < words.length - 1; i++) {
String cur = words[i];
String next = words[i + 1];
int smallerLen = Math.min(cur.length(), next.length()); for (int j = 0; j < smallerLen; j++) {
char c1 = cur.charAt(j);
char c2 = next.charAt(j); if (c1 != c2) {
//new set
Set<Character> set = new HashSet<>();
//contains c1
if (map.containsKey(c1)) set = map.get(c1); //not contain c2
if (!set.contains(c2)) {
set.add(c2);
map.put(c1, set);
degree.put(c2, degree.get(c2) + 1);
}
break;
}
}
} //bfs, get answer
Queue<Character> q = new LinkedList<>();
for (char c : degree.keySet()) {
if (degree.get(c) == 0) q.offer(c);
} while (!q.isEmpty()) {
char c1 = q.remove();
res += c1;
if (map.containsKey(c1)) {
for (char c2 : map.get(c1)) {
degree.put(c2, degree.get(c2) - 1);
if (degree.get(c2) == 0) q.offer(c2);
}
}
} //cc at end
if (res.length() != degree.size()) return ""; return res;
}
}

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