Reverse String II Problem & Solution

Given a string s and an integer k, reverse the first k characters for every 2k characters counting from the start of the string.

If there are fewer than k characters left, reverse all of them. If there are less than 2k but greater than or equal to k characters, then reverse the first k characters and left the other as original.

See the reverse string ii problem on LeetCode.

C++ Solution

#pragma GCC optimize("Ofast")
#pragma GCC optimization("unroll-loops")

static const int _=[](){ios::sync_with_stdio(false);cin.tie(nullptr);cout.tie(nullptr);return 0;}();

class Solution {
  string reverseStr(string s, int k) {
    for (int i = 0; i < s.size(); i += 2 * k) {
      reverse(s.begin() + i, min(s.end(), s.begin() + i + k));
    return s;

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