Find_pivot_integer.cpp

class Solution
{
public:
    int pivotInteger(int n)
    {
        int leftBound = 1, rightBound = n;
        int fullSum = (n * (n + 1)) / 2;
        while (leftBound <= rightBound)
        {
            int midNum = leftBound + (rightBound - leftBound) / 2;
            int firstHlfSum = (midNum * (midNum + 1)) / 2;
            int secondHlfSum = fullSum - firstHlfSum + midNum;
            if (firstHlfSum == secondHlfSum)
                return midNum;
            else if (firstHlfSum < secondHlfSum)
            {
                leftBound = midNum + 1;
            }
            else
            {
                rightBound = midNum - 1;
            }
        }
        return -1;
    }
};