边工作边刷题：70天一遍leetcode: day 16-3

Path Sum I/II

同上，preorder搞定。这题竟然犯了2个错误。
错误点

• pop不能发生在入左右子树之后
• 每个节点对应的sum值是减去该节点的val，所以root初始要减去val

II用递归做的。递归里，root不为None，而且在外层更新sum，内层只是中止条件判断（sum为0，左右为空）

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def hasPathSum(self, root, sum):
        """
        :type root: TreeNode
        :type sum: int
        :rtype: bool
        """
        if not root: return False
        stk = [(root, sum-root.val)]
        while stk:
            cur = stk[-1][0]
            s = stk[-1][1]
            stk.pop()
            if not cur.left and not cur.right and s==0:
                return True
                
            if cur.left:
                stk.append((cur.left, s-cur.left.val))
                
            if cur.right:
                stk.append((cur.right, s-cur.right.val))
            #print stk
        return False