算法-二分查找-1

普通二分查找

public int binarySearch(int[] arr, int target) {
        int l = 0;
        int r = arr.length - 1;
        int mid = l + ((r - l) >> 1);
        while (l <= r) {
            if (arr[mid] < target) {
                l = mid + 1;
            } else if (arr[mid] > target) {
                r = mid - 1;
            } else {
                return mid;
            }
        }
        return -1;
    }

找到第一次出现的目标值下标

public int search(int[] arr, int target) {
        int l = 0;
        int r = arr.length;
        while (l < r) {
            int mid = l + ((r - l) >> 1);
            if (arr[mid] == target) {
                r = mid;
            } else if (arr[mid] < target) {
                l = mid + 1;
            } else {
                r = mid;
            }
        }
        if (l == arr.length) {
            return -1;
        }
        return arr[l] == target ? l : -1;
    }

找到最后出现的目标值下标

public int search(int[] arr, int target) {
        int l = 0;
        int r = arr.length;
        while (l < r) {
            int mid = l + ((r - l) >> 1);
            if (arr[mid] == target) {
                l = mid + 1;
            } else if (arr[mid] < target) {
                l = mid + 1;
            } else {
                r = mid;
            }
        }
　　　　　　　　// 因为l比目标值后移一位所以判断l==arr.length不能使用 坐标也不可能到0
        if (l == 0) {
            return -1;
        }
        return arr[l - 1] == target ? l - 1 : -1;
    }