Permutation in string

Hash Table Two Pointers String Sliding Window

48.5% acceptance

Jan 13, 2026

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Given two strings s1 and s2, return true if s2 contains a permutation of s1, or false otherwise. In other words, return true if one of s1's permutations is the substring of s2.

Solution

Rust

Time O(n)

Space O(1)

LeetCode

solution.rs

impl Solution {
  pub fn check_inclusion(s1: String, s2: String) -> bool {
    let s1 = s1.as_bytes();
    let s2 = s2.as_bytes();
    let k = s1.len();
    if k > s2.len() { return false; }
    let mut need = [0i32; 26];
    let mut window = [0i32; 26];
    for &b in s1 { need[(b - b'a') as usize] += 1; }
    for i in 0..s2.len() {
      window[(s2[i] - b'a') as usize] += 1;
      if i >= k { window[(s2[i - k] - b'a') as usize] -= 1; }
      if window == need { return true; }
    }
    false
  }
}