def shiftDown(heap, startPos, pos):
    """
        Follow the path to the root, moving parents interval down until finding a place. 

            Parameters:
                    heap (List[List[int]]): array of meeting time intervals where intervals[i] = [starti, endi]
                    startPos (int): index of array interval initial position
                    pos (int): index of array interval we are trying to shift down
            Returns:
                    heap (int):  heap where the meeting time interval is moved up 

        >>> shiftDown([[7,10],[2,4]], 0, 1)
        [[2, 4], [7, 10]]
    """

    initialPos = heap[pos]
    while pos > startPos:
        parentPos = (pos - 1) // 2
        parentInterval = heap[parentPos]

        if initialPos[1] < parentInterval[1]:
            heap[pos] = parentInterval
            pos = parentPos
            continue
        break
    heap[pos] = initialPos

    return heap