Binary Tree Leaves Order Traversal

这道题需要观察到一个很关键的规律，即统一数组内的结点的高度是一样的，从这个角度再去思考算法就很容易想到利用哈希表收集同高度的结点即可，另外这里的同层结点顺序利用先序遍历即可，另外注意这里需要在最后对哈希表的key进行排序也就是按层数顺序输入结果

"""
Definition of TreeNode:
class TreeNode:
    def __init__(self, val):
        self.val = val
        self.left, self.right = None, None
"""
class Solution:
    # @param {TreeNode} root the root of binary tree
    # @return {int[][]} collect and remove all leaves
    def findLeaves(self, root):
        # Write your code here
        store = {}
        self.helper(root, store)
        result = []
        for key in sorted(store.keys()):
            result.append(store[key])
        return result

    def helper(self, root, store):
        if not root:
            return 0

        left = self.helper(root.left, store)
        right = self.helper(root.right, store)
        height = max(left, right) + 1
        store.setdefault(height, [])
        store[height].append(root.val)
        return height