遍历

倒序遍历，从后往前遍历即可

import java.util.*;


public class Solution {
    /**
     * 反转字符串
     * @param str string字符串 
     * @return string字符串
     */
    public String solve (String str) {
        int n = str.length();
        char strs[] = new char[n];
        for(int i = n - 1;i >= 0;i--){
            strs[i] = str.charAt(n - 1 - i);
        }
        return new String(strs);
    }
}