LeetCode 112. 路径总和

示例: 
给定如下二叉树，以及目标和 sum = 22，

              5
             / \
            4   8
           /   / \
          11  13  4
         /  \      \
        7    2      1
返回 true, 因为存在目标和为 22 的根节点到叶子节点的路径 5->4->11->2。

class Solution:
    def hasPathSum(self, root: TreeNode, sum: int) -> bool:
        if root == None:return False
        root.val = sum - root.val
        stack = []
        #先序遍历
        while root or len(stack) != 0:
            pre:TreeNode#保存移动之前的点，用于赋值
            if root:
                #扣除后刚好为0的叶节点符合条件
                if root.val == 0 and root.left == None and root.right == None:
                    return True
                stack.append(root)
                pre = root
                root = root.left
            else:
                root = stack.pop()
                pre = root
                root = root.right
            if root:
                root.val = pre.val - root.val
        return False