34. Find First and Last Position of Element in Sorted Array

对排序的数组查找目标值的左右边界。

ex. [5,7,7,8,8,10] , target 8 => [3,4].

思考：对于已经排序的数组，通常会想到二分插入，这道题使用二分查找的变体即可。分别找到左右边界。

    int LeftBound(vector &nums, int target)
    {
        int begin = 0;
        int end = nums.size() - 1;
        
        // 要有等于：可能是begin和end都指向同一个target位置，也可能指向非值为target的位置
        while(begin <= end)
        {
            int mid = (begin + end) / 2;
            if(target == nums[mid])
            {
                if(mid == 0 || target > nums[mid - 1])
                    return mid;
                end = mid - 1;
            }
            else if(target > nums[mid])
            {
                begin = mid + 1;
            }
            else
            {
                end = mid - 1;
            }
        }
        return -1;
    }
    
    int RightBound(vector &nums, int target)
    {
        int begin = 0;
        int end = nums.size() - 1;
        
        while(begin <= end)
        {
            int mid = (begin + end) / 2;
            if(target == nums[mid])
            {
                if(mid == nums.size() - 1 || target < nums[mid + 1])
                    return mid;
                begin = mid + 1;
            }
            else if(target > nums[mid])
            {
                begin = mid + 1;
            }
            else
            {
                end = mid - 1;
            }
        }
        return -1;
    }
    
    vector searchRange(vector& nums, int target) {
        vector v;
        v.push_back(LeftBound(nums, target));
        v.push_back(RightBound(nums, target));
        return v;
    }