Given a string and number K, find the substrings of size K with K distinct characters. If no, output empty list. Remember to emit the duplicate substrings, i.e. if the substring repeated twice, only output once.

  • 字符串中等题。Sliding window algorithm + Hash。
  • 使用移动窗口算法,两个指针标记window起点left/终点right,还有一个计数器count记录hit count,初始值为K。另外还有一个hash数组来记录当前window中所有char。
  • 注意hit的条件是hash[i] == 0,代表当前window中没有重复char。如果hit,则-- count代表找到一个满足条件char。
  • ++ hash[right]来标记已经在当前window中出现过,并扩展right。
  • 如果window size为K,那么就可以判断如果count == 0,代表已经找到K个不重复的char,可以放入结果集。这里注意下需要用STL算法find()去重。
  • 接着要把window往右移,同时对right char做过的操作进行恢复。
  • 注意如果hash[left] == 1,代表以前满足过hash[right] == 0,所以需要-- count来恢复。而对于hash[left] > 1,因为重复char只会hit一次,只会对count + 1,所以不需要-- count,只要等到hash[left] == 1的时候再count - 1就行。同时因为left要移出window了,所以-- hash[left]来恢复,并右移left扩展到下一个window。
  • find - C++ Reference
    • http://www.cplusplus.com/reference/algorithm/find/?kw=find
 //
// main.cpp
// LeetCode
//
// Created by Hao on 2017/3/16.
// Copyright © 2017年 Hao. All rights reserved.
// #include <iostream>
#include <vector>
#include <unordered_map>
using namespace std; class Solution {
public:
vector<string> subStringKDist(string S, int K) {
vector<string> vResult; // corner case
if (S.empty()) return vResult; unordered_map<char, int> hash; // window start/end pointer, hit count
int left = , right = , count = K; while (right < S.size()) {
if (hash[S.at(right)] == ) // hit the condition 1 dup char
-- count; ++ hash[S.at(right)]; // increase hash value to mark that the char exists in the current window ++ right; // move window end pointer rightward // window size reaches K
if (right - left == K) {
if ( == count) { // find K distinct chars
if (find(vResult.begin(), vResult.end(), S.substr(left, K)) == vResult.end()) // using STL find() to avoid dup
vResult.push_back(S.substr(left, K));
} // be careful for the restore condition. Count is only increased when hash[i] == 0, so only hash[i] == 1 means that count was increased.
if (hash[S.at(left)] == )
++ count; -- hash[S.at(left)]; // decrease to restore hash value ++ left; // move window start pointer rightward
}
} return vResult;
}
}; int main(int argc, char* argv[])
{
Solution testSolution; vector<string> sInputs = {"awaglknagawunagwkwagl", "abccdef", "", "aaaaaaa"};
vector<int> iInputs = {, , , };
vector<string> result; /*
{wagl aglk glkn lkna knag gawu awun wuna unag nagw agwk kwag }
{ab bc cd de ef }
{}
{}
*/
for (auto i = ; i < sInputs.size(); ++ i) {
result = testSolution.subStringKDist(sInputs[i], iInputs[i]); cout << "{";
for (auto it : result)
cout << it << " ";
cout << "}" << endl;
} return ;
}

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