113_Path Sum II

113. Path Sum II

Level: medium

Tag: tree, dfs

Question

Given a binary tree and a sum, find all root-to-leaf paths where each path's sum equals the given sum.

Example 1

Given the below binary tree and sum = 22,
              5
             / \
            4   8
           /   / \
          11  13  4
         /  \    / \
        7    2  5   1

return

[
   [5,4,11,2],
   [5,8,4,5]
]

Solution 1: recursive (w/o defining a helper function)

# 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 pathSum(self, root, sum):
        """
        :type root: TreeNode
        :type sum: int
        :rtype: List[List[int]]
        """
        res = []

        if not root:
            return res

        sum -= root.val

        if not root.left and not root.right:
            if sum == 0:
                res.append([root.val])

        if root.left: 
            for path in self.pathSum(root.left, sum):
                res.append([root.val] + path)

        if root.right:
            for path in self.pathSum(root.right, sum):
                res.append([root.val] + path)

        return res

Solution 2: iterative (Depth first search, using stack)

# 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 pathSum(self, root, sum):
        """
        :type root: TreeNode
        :type sum: int
        :rtype: List[List[int]]
        """
        if not root:
            return []

        res = []
        stack = [(root, sum, [])]

        while stack:
            root, sum, path_list = stack.pop()
            sum -= root.val
            if not root.left and not root.right: 
                if sum == 0:
                    res.append(path_list + [root.val])
            if root.right:
                stack.append((root.right, sum, path_list + [root.val]))
            if root.left:
                stack.append((root.left, sum, path_list+ [root.val]))
        return res