面试题53 - I. 统计一个数字在排序数组中出现的次数

题目：

 

 

解答：

class Solution {
public:
    int search(vector<int>& nums, int target) 
    {
        int left = left_bound(nums, target);
        int right = right_bound(nums, target);

        if (left == -1 || right == -1)
        {
            return 0;
        }
        
        return right - left + 1;
    }

    int left_bound(vector<int> nums, int target) 
    {
        int left = 0;
        int right = nums.size() - 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，收缩左侧边界
                right = mid - 1;
            }
        }
        
        // 最后要检查 left 越界的情况
        if (left >= nums.size() || nums[left] != target)
        {
            return -1;
        }
        return left;
    }


    int right_bound(vector<int> nums, int target) 
    {
        int left = 0;
        int right = nums.size() - 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，收缩右侧边界
                left = mid + 1;
            }
        }
        // 最后要检查 right 越界的情况
        if (right < 0 || nums[right] != target)
        {
            return -1;
        }
        return right;
    }
};