Java二分法总结

一：用法

1、有序、无重复的数组
2、当有重复时，可以先用二分法查找到，然后使用左右滑动来找到边界
3、二分法例子：

• 给定一个n个元素有序的（升序）整型数组nums和一个目标值target，写一个函数搜索nums中的target，如果目标值存在返回下标，否则返回-1。

public class Search {
    public static void main(String[] args) {
        int[] nums = {-1, 0, 3, 5, 9, 12};
//        int target = 9;
        int target = 2;

        //方法一：暴力循环
        int i = method1(nums, target);
        System.out.println(i);

        //方法二：target【】双闭区间
        int get2 = method2(nums, target);
        System.out.println(get2);

        //方法三：【）左闭右开区间
        int get3 = method3(nums, target);
        System.out.println(get3);
    }

    private static int method3(int[] nums, int target) {

        int left = 0;
        int right = nums.length;//[)
        while (left < right) {
            int mid = left + ((right - left) >> 1);
            if (nums[mid] > target) {
                right = mid;
            } else if (nums[mid] < target) {
                left =mid+1;
            } else if (nums[mid] == target) {
                return mid;
            }
        }
        return -1;


    }

    private static int method2(int[] nums, int target) {

        int left = 0;
        int right = nums.length - 1; //因为此处是-1，所以要用双闭区间
        while (left <= right) {
            int mid = left + ((right - left) >> 1);
            if (nums[mid] > target) {
                right = mid - 1;
            } else if (nums[mid] < target) {
                left = mid + 1;
            } else if (nums[mid] == target) {
                return mid;
            }
        }
        //未找到目标
        return -1;

    }

    private static int method1(int[] nums, int target) {
        int get = -1;
        for (int i = 0; i < nums.length; i++) {
            if (nums[i] == target) {
                get = i;
                break;
            }
        }
        return get;
    }
}

二：区间

2.1：