数组反转

一个数组int[] numArr = {0,1,2,3,4,5,6,7,8,9}，将其进行倒叙排列为{9,8,7,6,5,4,3,2,1,0}，也就是将数组进行反转。这里分列两种方式：由外入里、由里到外。算法的关键在于确定交换的元素边界，当数组为偶数时，头尾交换正适合，而为奇数时，则存在中间元素。

• 由外到里
  1、变量交换，通过中间变量num1,num2进行预存numArr[i]和numArr[len - 1 - i],然后进行交换numArr[i]和numArr[len - 1 - i]。

public static void swap1(int[] numArr) {
        final int len = numArr.length;
        int count, num1, num2, index;
        if(len % 2 == 0) {
            count = len / 2;
        } else {
            count = (len - 1) / 2;
        }
        for(int i = 0; i < count; i++) {
            index = len - 1 - i;
            num1 = numArr[i];
            num2 = numArr[index];

            numArr[i] = num2;
            numArr[index] = num1;
        }
    }

2、不同中间变量交换两个数组元素

public static void swap2(int[] numArr) {
        final int len = numArr.length;
        int count, index;
        if(len % 2 == 0) {
            count = len / 2;
        } else {
            count = (len - 1) / 2;
        }
        for(int i = 0; i < count; i++) {
            index = len - 1 - i;
            numArr[i] = numArr[i] ^ numArr[index];
            numArr[index] = numArr[i] ^ numArr[index];
            numArr[i] = numArr[i] ^ numArr[index];
        }
    }

• 由里到外
  在上面的循环之中，数组下标由小到大，所以由外到里进行交换，而换一种方式，数组小标由大到小，则是由里到外。
  1、变量交换

    public static void swap3(int[] numArr) {
        final int lastIndex = numArr.length - 1;
        int num1, num2, index = lastIndex - 1 >> 1;
        for(; index >= 0; index--) {
            num1 = numArr[index];
            num2 = numArr[lastIndex - index];
            numArr[index] = num2;
            numArr[lastIndex - index] = num1;
        }   
        printArr(numArr);
    }

2、不同中间变量交换两个数组元素

public static void swap4(int[] numArr) {
        final int lastIndex = numArr.length - 1;
        for(int index = lastIndex - 1 >> 1; index >= 0; index--) {
            numArr[index] = numArr[index] ^ numArr[lastIndex - index];
            numArr[lastIndex - index] = numArr[index] ^ numArr[lastIndex - index];
            numArr[index] = numArr[index] ^ numArr[lastIndex - index];
        }   
        printArr(numArr);
    }

总结

数组元素反转关键在于for循坏边界确定，同时优化元素变量交换，通过异或运算符来减少变量的使用，此外lastIndex - 1右移1位等同于除以2，而lastIndex作为numArr数组元素最后下标，当为奇数时，可以减少一次交换，即numArr[index] ==numArr[lastIndex - index]。