leetcode 451. Sort Characters By Frequency
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Medium
Given a string, sort it in decreasing order based on the frequency of characters.
Example 1:
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:
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:
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.
class Solution { public: string frequencySort(string s) { int len = s.size(); // 建立一个hash表 unordered_map<char, int> hash; // 建立频率和字符的对应关系 vector<string> vec(len+1, ""); for(auto ch: s) hash[ch]++; for(auto val: hash) vec[val.second].append(val.second, val.first); string ans; for(int i = len; i > 0; i--) ans += vec[i]; return ans; } };
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