Introduction

In this blog post, we will explore how to solve the LeetCode problem "113" using AI. LeetCode is a popular platform for preparing for coding interviews, and with the help of AI tools like Chatmagic, we can generate solutions quickly and efficiently - helping you pass the interviews and get the job offer without having to study for months.

Problem Statement

Given the root of a binary tree and an integer targetSum, return all root-to-leaf paths where the sum of the node values in the path equals targetSum. Each path should be returned as a list of the node values, not node references. A root-to-leaf path is a path starting from the root and ending at any leaf node. A leaf is a node with no children. Example 1: Input: root = [5,4,8,11,null,13,4,7,2,null,null,5,1], targetSum = 22 Output: [[5,4,11,2],[5,8,4,5]] Explanation: There are two paths whose sum equals targetSum: 5 + 4 + 11 + 2 = 22 5 + 8 + 4 + 5 = 22 Example 2: Input: root = [1,2,3], targetSum = 5 Output: [] Example 3: Input: root = [1,2], targetSum = 0 Output: [] Constraints: The number of nodes in the tree is in the range [0, 5000]. -1000 <= Node.val <= 1000 -1000 <= targetSum <= 1000

Explanation

Here's an efficient solution to the problem:

• Depth-First Search (DFS): Traverse the tree using DFS, exploring each possible path from the root to a leaf.
• Path Tracking: Maintain a list to track the current path being explored.

• Sum Calculation: Keep track of the current path's sum as we traverse. When a leaf node is reached, check if the current sum equals the targetSum. If it does, add a copy of the current path to the result.

• Time Complexity: O(N), where N is the number of nodes in the tree.

• Space Complexity: O(H), where H is the height of the tree (in the worst case, a skewed tree where H = N). This is due to the recursion stack and the space used to store the paths.

Code

    from typing import List, Optional

# Definition for a binary tree node.
class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

class Solution:
    def pathSum(self, root: Optional[TreeNode], targetSum: int) -> List[List[int]]:
        result = []
        path = []

        def dfs(node: Optional[TreeNode], current_sum: int):
            if not node:
                return

            path.append(node.val)
            current_sum += node.val

            if not node.left and not node.right:  # Leaf node
                if current_sum == targetSum:
                    result.append(path.copy())  # Append a copy to avoid modification
            else:
                dfs(node.left, current_sum)
                dfs(node.right, current_sum)

            path.pop()  # Backtrack

        dfs(root, 0)
        return result