leetcode 34. Search for a Range

二分查找算法在找到目标值后的边界处理

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本文详细介绍了如何使用二分查找算法在已排序的整数数组中找到给定目标值的起始和结束位置。通过示例说明了在找到目标值后，如何进一步扩展查找范围以确定其边界。

1.题目

Given a sorted array of integers, find the starting and ending position of a given target value.

Your algorithm's runtime complexity must be in the order of O(log n).

If the target is not found in the array, return [-1, -1].

For example,
Given [5, 7, 7, 8, 8, 10] and target value 8,
return [3, 4].

2.思路

二分查找，然后向两边延伸。。。

class Solution {
public:
    int bSearch(vector<int> &nums,int target){
        int left = 0,right = nums.size()-1,mid = 0;
        while(left <= right){
            mid = (right - left)/2 + left;
            if(nums[mid] == target)
                return mid;
            else if(nums[mid] > target)
                right = mid - 1;
            else
                left = mid + 1;
        }
        return -1;
    }
    vector<int> searchRange(vector<int>& nums, int target) {
        vector<int> ans(2,-1);
        int index = bSearch(nums,target);
        if(index == -1) return ans;
        else{
            int left = index,right = index;
            while(left >=0 && nums[left] == target)
                left--;
            while(right < nums.size() && nums[right] == target)
                right++;
            ans[0] = left+1;
            ans[1] = right-1;
            return ans;
        }
    }
};