LeetCode #112 Path Sum

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Problem

Given a binary tree and a sum, determine if the tree has a root-to-leaf path such that adding up all the values along the path equals the given sum.

Example

Given the below binary tree and sum = 22,

1
2
3
4
5
6
7
      5
     / \
    4   8
   /   / \
  11  13  4
 /  \      \
7    2      1

return true, as there exist a root-to-leaf path 5->4->11->2 which sum is 22.

Note

A leaf is a node with no children.

Solution

Method: Depth-first Search

Time Complexity: O(n)

Space Complexity: O(n)

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

class Solution:
    def hasPathSum(self, root, sum):
        """
        :type root: TreeNode
        :type sum: int
        :rtype: bool
        """
        stack = [(root, 0, False)]
        
        while stack:
            node, pre_val, visited = stack.pop()
            
            if node:
                if not visited:
                    stack.append((node, pre_val, True))
                    pre_val += node.val

                    if pre_val == sum and \
                       not node.left and \
                       not node.right:
                        return True

                    stack.append((node.left, pre_val, False))
                    stack.append((node.right, pre_val, False))
        
        return False