[Leetcode] 451. Sort Characters By Frequency 解题报告

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题目：

Given a string, sort it in decreasing order based on the frequency of characters.

Example 1:

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Input:
"tree"

Output:
"eert"

Explanation:
'e' appears twice while 'r' and 't' both appear once.
So 'e' must appear before both 'r' and 't'. Therefore "eetr" is also a valid answer.

Example 2:

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Input:
"cccaaa"

Output:
"cccaaa"

Explanation:
Both 'c' and 'a' appear three times, so "aaaccc" is also a valid answer.
Note that "cacaca" is incorrect, as the same characters must be together.

Example 3:

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Input:
"Aabb"

Output:
"bbAa"

Explanation:
"bbaA" is also a valid answer, but "Aabb" is incorrect.
Note that 'A' and 'a' are treated as two different characters.

思路：

这道题目如果采用基于比较的排序，则思路也很简单，不过时间复杂度最少需要O(nlogn)。但是如果采用计数排序，则可以将时间复杂度降低到O(n)。所以这道题目的考点我感觉其实是计数排序，请参考下面的代码。

代码：

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class Solution {
public:
    string frequencySort(string s) {
        int len = s.size();
        unordered_map<char, int> hash;
        for(auto ch : s) {
            ++hash[ch];
        }
        vector<string> vec(len + 1, "");
        for(auto val : hash) {
            vec[val.second].append(val.second, val.first);
        }
        string ret;
        for(int i = len; i >= 0; --i) {
            ret += vec[i];
        }
        return ret;
    }
};