Leetcode•Jul 23, 2025

Buddy Strings

Hazrat Ali

Leetcode

Given two strings s and goal, return true if you can swap two letters in s so the result is equal to goal, otherwise, return false.

Swapping letters is defined as taking two indices i and j (0-indexed) such that i != j and swapping the characters at s[i] and s[j].

  • For example, swapping at indices 0 and 2 in "abcd" results in "cbad".

 

Example 1:

Input: s = "ab", goal = "ba"
Output: true
Explanation: You can swap s[0] = 'a' and s[1] = 'b' to get "ba", which is equal to goal.

Example 2:

Input: s = "ab", goal = "ab"
Output: false
Explanation: The only letters you can swap are s[0] = 'a' and s[1] = 'b', which results in "ba" != goal.

Example 3:

Input: s = "aa", goal = "aa"
Output: true
Explanation: You can swap s[0] = 'a' and s[1] = 'a' to get "aa", which is equal to goal.


Solution
/**
 * @param {string} A
 * @param {string} B
 * @return {boolean}
 */
const buddyStrings = (A, B) => {
  if (!A || !B || A.length !== B.length) {
    return false;
  }

  if (A === B) {
    const count = {};

    for (let c of A) {
      count[c] = ~~count[c] + 1;
    }

    for (let c in count) {
      if (count[c] > 1) {
        return true;
      }
    }

    return false;
  }

  let first = -1;
  let second = -1;

  for (let i = 0; i < A.length; i++) {
    if (A[i] !== B[i]) {
      if (first === -1) {
        first = i;
      } else if (second === -1) {
        second = i;
      } else {
        return false;
      }
    }
  }

  return second !== -1 && A[first] === B[second] && A[second] === B[first];
};



Comments