Reverse String II

题目地址：
https://leetcode.com/problems/reverse-string-ii/description/

题目：
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:

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

解题思路：
这道题就是stringbuilder和双指针问题。


代码：

public String reverseStr(String s, int k) {
    if(s == null || s.length() <= 1){
        return s;
    }
    StringBuilder sb = new StringBuilder();
    if(s.length() <= k){
        int i = s.length() - 1;
        while(i >= 0){
            sb.append(s.charAt(i));
            i -= 1;
        }
    }
    else{
        int i = 1;
        while(k * i - 1 <= s.length() - 1){
            if(i % 2 == 1){
                // reverse here                int start = (i - 1) * k;
                int end = i * k - 1;
                reverse(sb, s, start, end, true);
            }
            else{
                // directly append                int start = (i - 1) * k;
                int end = i * k - 1;
                reverse(sb, s, start, end, false);
            }
            i++;
        }
        if(i * k - 1 != s.length() - 1){
            if((i - 1) % 2 == 0){
                // reverse the rest                int start = (i - 1) * k;
                int end = s.length() - 1;
                reverse(sb, s, start, end, true);
            }
            else{
                // directly append the rest                int start = (i - 1) * k;
                int end = s.length() - 1;
                reverse(sb, s, start, end, false);
            }
        }
    }
    return sb.toString();
}

private void reverse(StringBuilder sb, String s, int start, int end, boolean reverse) {
    if(reverse){
        while(start <= end){
            sb.append(s.charAt(end));
            end--;
        }
    }
    else{
        while(start <= end){
            sb.append(s.charAt(start));
            start++;
        }
    }
}

public static void main(String[] args){
    ReverseString2 reverseString2 = new ReverseString2();
    String s = "abcdefg";
    String rst = reverseString2.reverseStr(s, 2);
    System.out.println(rst);
}