1，题目

2，思路

• 用List集合来装字符串其中每k个为一个元素单位
• 我们根据题目意思就可以明白list中偶数位需要反转reverse，奇数保持原样
• 再全部拼接一块
• 最后return tostring

3，代码

import java.util.ArrayList;
import java.util.List;

public class Leetcode541 {
    public static void main(String[] args) {
        System.out.println(new Solution541().reverseStr("abcdefg", 2));//"bacdfeg"
    }
}

class Solution541 {
    public String reverseStr(String s, int k) {
        List<StringBuffer> list = new ArrayList<>();
        int i = 0;
        int j = k;
        for (; j < s.length(); i = j, j += k) {
            list.add(new StringBuffer(s.substring(i, j)));//满足k条件的字符串
        }
        list.add(new StringBuffer(s.substring(i)));//截取剩余的字符串
        for (i = 0; i < list.size(); i++) {
            if (i % 2 == 0) list.get(i).reverse();//偶数次反转
        }
        StringBuffer stringBuffer = new StringBuffer();
        for (StringBuffer buffer : list) {
            stringBuffer.append(buffer);
        }
        return stringBuffer.toString();
    }
}