Reverse String II | LeetCode 541 | C

2023-09-21
#LeetCode #C #String #Two Pointers
LeetCode

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 leave the other as original.

Example 1:

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Input: s = "abcdefg", k = 2
Output: "bacdfeg"

Example 2:

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Input: s = "abcd", k = 2
Output: "bacd"

Constraints:

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1 <= s.length <= 104
s consists of only lowercase English letters.
1 <= k <= 104

Approach

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Use two pointers to reverse substring every 2k.

Algorithm

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1. Go through the list
2. Reverse k characters every 2k.
3. Always check if the right pointer exceeds the lenght-1.

Implementation

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char * reverseStr(char * s, int k){
  int gap = 2*k;
  int left, right, len = strlen(s);
  char c = '\0';
  for(int i = 0;i<len;i+=gap)
  {
    left = i;
    right = (i+k<=len-1)?(i+k-1):(len-1);
    while(left<right){
      c = s[left];
      s[left] = s[right];
      s[right] = c;
      left++;
      right--;
    }
  }
  return s;
}