二分查找法

1，循环实现

public class 二分查找法_01 {

    public static void main(String[] args) {
        long start = System.currentTimeMillis();
        int[] arr = new int[100000000];
        for (int i = 0; i < 100000000; i++) {
            arr[i] = i;
        }

        二分查找法_01 instance = new 二分查找法_01();
        Integer result = instance.find(arr,17);
        System.out.println(result);
        long end = System.currentTimeMillis();

        System.out.println("花费时间 = " + (end - start) );
    }

    public Integer find(int[] arr, int target){

        int low = 0;
        int high = arr.length;

        int count = 0;

        while (low <= high){
            count++;
            System.out.println("查找次数" + count);

            int mid = (low + high) / 2;
            int guess = arr[mid];

            if(target == guess){
                return guess;
            }

            if(target < guess){
                high = mid - 1;
            }else {
                low = mid + 1;
            }
        }

        return null;
    }
}

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2，递归方式实现

public Integer find(int[] arr, int low, int high, int target){

        if(low <= high){
            int mid = (low + high)/2;
            int guess = arr[mid];
            if(target < guess){
                return find(arr, low, mid - 1, target);
            }

            if(target > guess){
                return find(arr, mid + 1, high, target);
            }

            return guess;
        }

        return null;
    }

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两种算法：

1. mid=(low+high)/2
2. mid=low+(high-low)/2
   第二种可防溢出，保证计算出来的mid，一定大于low，小于high，不存在溢出的问题。

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二分查找法的缺点：

因为需要有序数组，但是数组的删除插入效率不高，在无序数组构建有序数组的过程中效率较低，
可使用平衡二叉树、二叉树