小知识，大挑战！本文正在参与“程序员必备小知识”创作活动。
描述
Given the root of a binary tree and an integer targetSum, return true if the tree has a root-to-leaf path such that adding up all the values along the path equals targetSum.
A leaf is a node with no children.
Example 1:
Input: root = [5,4,8,11,null,13,4,7,2,null,null,null,1], targetSum = 22
Output: true
Example 2:
Input: root = [1,2,3], targetSum = 5
Output: false
Example 3:
Input: root = [1,2], targetSum = 0
Output: false
Note:
The number of nodes in the tree is in the range [0, 5000].
-1000 <= Node.val <= 1000
-1000 <= targetSum <= 1000
解析
根据题意，就是给出了一个二叉树的根节点 root 和一个目标整数 targetSum ，如果这棵树有一条从根节点到叶子节点的路径的和等于 targetSum ，就返回 true ，否则返回 false 。
仍然使用递归解法，定义一个递归函数 dfs ，让其去深度遍历从 root 到某个 leaf 的路径存入列表 nodes 中，当到达叶子节点的时候比较列表 nodes 中的值的和是否等于 targetSum 。
解答
class TreeNode(object):
def __init__(self, val=0, left=None, right=None):
self.val = val
self.left = left
self.right = right
class Solution(object):
def hasPathSum(self, root, targetSum):
"""
:type root: TreeNode
:type targetSum: int
:rtype: bool
"""
if not root:return False
def dfs(root, nodes):
if not root: return sum(nodes) == targetSum
if not root.left:
return dfs(root.right, nodes+[root.val])
if not root.right:
return dfs(root.left, nodes+[root.val])
if root.left and root.right:
return dfs(root.right, nodes+[root.val]) or dfs(root.left, nodes+[root.val])
return dfs(root, [])
运行结果
Runtime: 28 ms, faster than 94.13% of Python online submissions for Path Sum.
Memory Usage: 15.4 MB, less than 51.88% of Python online submissions for Path Sum.
解析
其实也可以不定义其他的递归函数，直接在 hasPathSum 函数上进行递归，原理和上面一样。
解答
class TreeNode(object):
def __init__(self, val=0, left=None, right=None):
self.val = val
self.left = left
self.right = right
class Solution(object):
def hasPathSum(self, root, targetSum):
"""
:type root: TreeNode
:type targetSum: int
:rtype: bool
"""
if not root:return False
if not root.left and not root.right and root.val==targetSum:
return True
targetSum -= root.val
return self.hasPathSum(root.left, targetSum) or self.hasPathSum(root.right, targetSum)
运行结果
Runtime: 65 ms, faster than 5.78% of Python online submissions for Path Sum.
Memory Usage: 15.1 MB, less than 97.45% of Python online submissions for Path Sum.
原题链接：leetcode.com/problems/pa…
您的支持是我最大的动力
