Path Sum

题目描述

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.

For example: Given the below binary tree and sum = 22,

              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.

解题方法

DFS

DFS, 在stack里放一个tuple (node, curSum)

• node就是当前的treeNode
• curSum是到这个node为止的路径的sum

如果node是叶子节点，并且curSum == sum的时候，就说明有这样一条路径

recursion

recursion的方法就是将sum减去当前node的value,然后不断向下传递

Solution

# 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

        stack = []
        stack.append((root, root.val))
        while stack:
            curNode, curSum = stack.pop()
            if not curNode.left and not curNode.right and curSum == sum:
                return True
            if curNode.left:
                stack.append((curNode.left, curSum + curNode.left.val))
            if curNode.right:
                stack.append((curNode.right, curSum + curNode.right.val))

        return False

recursion

# 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

        if not root.left and not root.right and root.val == sum:
            return True

        return self.hasPathSum(root.left, sum-root.val) or self.hasPathSum(root.right, sum-root.val)

Reference