二分查找学习

直接看寻找一个数、寻找左侧边界、寻找右侧边界的代码。
即可了解解题基本套路。直接返回，或者锁定边界并且检查越界。

int binary_search(int[] nums, int target) {
    int left = 0, right = nums.length - 1; 
    while(left <= right) {
        int mid = left + (right - left) / 2;
        if (nums[mid] < target) {
            left = mid + 1;
        } else if (nums[mid] > target) {
            right = mid - 1; 
        } else if(nums[mid] == target) {
            // 直接返回
            return mid;
        }
    }
    // 直接返回
    return -1;
}

int left_bound(int[] nums, int target) {
    int left = 0, right = nums.length - 1;
    while (left <= right) {
        int mid = left + (right - left) / 2;
        if (nums[mid] < target) {
            left = mid + 1;
        } else if (nums[mid] > target) {
            right = mid - 1;
        } else if (nums[mid] == target) {
            // 别返回，锁定左侧边界
            right = mid - 1;
        }
    }
    // 最后要检查 left 越界的情况
    if (left >= nums.length || nums[left] != target)
        return -1;
    return left;
}


int right_bound(int[] nums, int target) {
    int left = 0, right = nums.length - 1;
    while (left <= right) {
        int mid = left + (right - left) / 2;
        if (nums[mid] < target) {
            left = mid + 1;
        } else if (nums[mid] > target) {
            right = mid - 1;
        } else if (nums[mid] == target) {
            // 别返回，锁定右侧边界
            left = mid + 1;
        }
    }
    // 最后要检查 right 越界的情况
    if (right < 0 || nums[right] != target)
        return -1;
    return right;
}