二分查找

int (vector<int> &nums, int target) {
    
    int n = nums.size();
    int first=0;
    int last =n;
    if(n=0){return -1;}
    int mid =(first+last)/2;
    while(first+1<last){
        if(nums[mid]>target){
            last = mid;
            mid =(first+last)/2;
        }
        else if(nums[mid]<target){
            first = mid;
            mid =(first+last)/2;
        }
        else if(nums[mid]=target){
            return mid;
            break;
        }
    }
    return -1;
}