时间复杂度（O(log(n))）,思想，二分查找，

class Solution {
public:
    vector<int> searchRange(vector<int>& nums, int target) {
        vector<int> vec;
        int index = triSearch(nums,0,nums.size()-1,target);
        if(index==-1){
            vec.push_back(-1);vec.push_back(-1);    
            return vec;
        }
        int start = index;
        int end = index;
        for(int i=index-1;i>=0;--i)
            if(nums[i]==target)start = i;
            else break;
        for(int j=index+1;j<nums.size();++j)
            if(nums[j]==target)end = j;
            else break;
        vec.push_back(start);
        vec.push_back(end);    
        return vec;
    }
    int triSearch(vector<int> nums,int start,int end,int k){
        while(start<=end)
            if(nums[(start+end)/2]==k)return (start+end)/2;
            else if(nums[(start+end)/2]>k)    end = (start+end)/2-1;
            else start = (start+end)/2+1;
        return -1;
    }
};