find 3 numbers in one array [1...n] to make a+b=c.

find 3 numbers in one array [1...n] to make a+b=c.
besides hash table method, we can also do it like this
The O(n^2) solution is a bit less intuitive - you have to solve the "2SUM" problem in O(n) for a sorted set.

bool ThreeSum(vector nums)
{
    std::sort(nums.begin(), nums.end());
    for (size_t i = 0; i < nums.size(); i++)
    {
        size_t j = 0;
        size_t k = nums.size() - 1;
        while ( j < k )
        {
            int result = nums[j] + nums[k] - nums[i];
            if ( (result < 0) || (i == j) )
                j++;
            else if ( (result > 0) || (i == k) )
                k--;
            else
                return true;
        }
    }
    return false;
}