leetcode 34在排序数组中查找元素的第一个和最后一个位置

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