［LeetCode］Path Sum II

太简单，不解释 

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

class Solution:
    # @param {TreeNode} root
    # @param {integer} sum
    # @return {integer[][]}
    def pathSum(self, root, sum):
        if not root:
            return []

        res = []
        self.dfs(root, sum, [], res)
        return res

    def dfs(self, root, sum, ls, res):
        if not root.left and not root.right:
            if root.val == sum:
                ls.append(root.val)
                res.append(ls)
        if root.left:
            self.dfs(root.left, sum-root.val, ls+[root.val], res)
        if root.right:
            self.dfs(root.right, sum-root.val, ls+[root.val], res)