Check if One String Swap Can Make Strings Equal

Scan both strings once and retain at most the two indices where characters differ. If lengths differ, the answer is false. With zero mismatches, it is true because at most one swap is allowed. With exactly two mismatches i and j, it is true precisely when s1[i] == s2[j] and s1[j] == s2[i]; swapping either string at those indices then aligns both characters. One mismatch can never be repaired by a swap, and three or more cannot be repaired by a single swap. Stop or mark failure once a third mismatch is found. This is clearer than constructing swapped copies and naturally handles repeated characters. Time is O(n), space is O(1) besides two indices.