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

public class ReverseElements {
    
    // 双指针 ArrayList 反转元素 出自 java.util.Collections#reverse
    public static void array() {
        List<Integer> list = new ArrayList<>(Arrays.asList(1, 2, 3, 4, 5, 6, 7));
        int size = list.size();
        Integer x, y;
        for (int begin = 0, mid = size >> 1, end = size - 1; begin < mid; begin++, end--) {
            x = list.get(begin);
            y = list.get(end);
            list.set(begin, y);
            list.set(end, x);
        }
        System.out.println(list);
    }

    // 双指针反转字符串 String无法直接操作内部的char数组 避免不了2倍内存开销
    public static void main(String[] args) {
        String str = "1234567";
        char[] chars = str.toCharArray();
        int len = chars.length;
        char x, y;
        for (int index = 0, mid = len >> 1, end = len - 1; index < mid; index++, end--) {
            x = chars[index];
            y = chars[end];
            chars[index] = y;
            chars[end] = x;
        }
        System.out.println(new String(chars));
    }

}