【算法】字符串反转

基本思路是找中间位置，然后镜像调换。
需要注意的是，以字符串长度/2做中间值和以字符数组最大索引做中间值的区别。

1，递归法

    public static String swap1(String str) {
        if (null == str) {
            return str;
        }

        char[] oldArr = str.toCharArray();
        return recursive(oldArr, 0);
    }

    private static String recursive(char[] oldArr, int i) {
        int indexMax = oldArr.length - 1;
        int half = (indexMax - 1) >> 1;

        if (i > half) {
            return new String(oldArr);
        } else {
            char a = oldArr[i];
            oldArr[i] = oldArr[indexMax - i];
            oldArr[indexMax - i] = a;
            return recursive(oldArr, ++i);
        }
    }

2, 非递归法

    public static String swap(String str) {
        if (null == str) {
            return str;
        }
        int len = str.length();
        char[] oldArr = str.toCharArray();

        int index = len - 1;
        int half = (index - 1) >> 1;

        for (int i = 0; i <= half; i++) {
            char a = oldArr[i];
            oldArr[i] = oldArr[index - i];
            oldArr[index - i] = a;
        }

        return new String(oldArr);
    }

3, 利用栈来实现
入栈（push）出栈（pop）即可