[leetcode]541. Reverse String II

Given a string and an integer k, you need to reverse the first k characters for every 2k characters counting from the start of the string. If there are less 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.

Example:

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

Restrictions: The string consists of lower English letters only.Length of the given string and k will in the range [1, 10000]

/** * @author binkang * @date May 7, 2017 */ public class ReverseString2 { public String reverseStr(String s, int k) { StringBuilder sb = new StringBuilder(); int start = 0; while(start <= s.length()){ int i = 0,j = 0; if(s.length() - start <= k){ i = s.length() - 1; j = s.length(); }else{ i = start + k - 1; j = start + k; } for( ;i >= start; i--) sb.append(s.charAt(i)); for(; j < s.length() && j < start + 2*k; j ++)sb.append(s.charAt(j)); start += 2*k; } return sb.toString(); } }