经典查找——二分查找

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Binary Search

二分查找，针对的是有序表 有序表 有序表

public static void main(String[] args) {
    int[] arr = {1,2,3,4,5,6,7,8,9,10};
    int  key = 7;
    int index = binarySearch(arr,key);
    System.out.println(index);
}

public static int binarySearch(int[] arr,int key) {
        int l = 0;
        int r = arr.length-1;
        int mid = 0;
        if(key < arr[l] || key > arr[r] || l > r){
            return -1;              
        }
        while(l<=r) {//这里必须是l<=r,如果只是小于会出错，甚至会查不到某些值
            mid = (l+r)/2;
            if(key < arr[mid]) {
                r = mid-1;
            }else if(key > arr[mid]) {
                l = mid+1;
            }else {
                return mid;
            }
        }
        return -1;
    }