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LeetCode Hot 100 P438 Find All Anagrams in a String

(Updated: )
algorithm
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2026-03-11

init

sliding window

Title:

P438 Find All Anagrams in a String

https://leetcode.cn/problems/find-all-anagrams-in-a-string/?envType=study-plan-v2&envId=top-100-liked

Classic sliding window problem

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// 2026.3.11 10:58
#include <vector>
#include <string>

using std::string;
using std::vector;

class Solution {
    public:
        vector<int> findAnagrams(string s, string p)
        {
                int i, index, valid = 0, need_valid = 0;
                int left = 0, right;
                int slen = s.size(), plen = p.size();
                vector<int> need(26, 0);
                vector<int> curr(26, 0);
                vector<int> ret;

                for (i = 0; i < plen; i++)
                        need[p[i] - 'a']++;
                
                for (i = 0; i < 26; i++)
                        if (need[i] > 0)
                                need_valid++;

                for (right = 0; right < slen; right++) {
                        if (right - left + 1 > plen) {
                                index = s[left] - 'a';

                                if (curr[index] == need[index])
                                        valid--;

                                curr[index]--;

                                left++;
                        }

                        index = s[right] - 'a';
                        curr[index]++;

                        if (curr[index] == need[index])
                                valid++;

                        if (valid == need_valid)
                                ret.push_back(left);
                }
                return ret;
        }
};
// 2026.3.11 11:19

acm mode rewrite:

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#include <iostream>
#include <string>
#include <unordered_map>
using namespace std;

int main()
{
        int n;
        string s, p;
        cin >> s;
        cin >> p;
        n = s.size();

        unordered_map<char, int> need, curr;
        for (char ch : p)
                need[ch]++;

        int left = 0, right = 0;
        int curr_valid = 0, need_valid = need.size();

        while (right < n) {
                char ch = s[right];
                curr[ch]++;

                if (curr[ch] == need[ch])
                        curr_valid++;

                right++;

                while (curr_valid == need_valid) {
                        char ch = s[left];
                        if (right - left == p.size())
                                cout << left << ' ';

                        if (curr[ch] == need[ch])
                                curr_valid--;
                        
                        curr[ch]--;
                        left++;
                }
        }
}