Binary Tree Paths

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LintCode: https://www.lintcode.com/en/problem/binary-tree-paths/

LeetCode: https://leetcode.com/problems/binary-tree-paths/#/description

Question

Given a binary tree, return all root-to-leaf paths.

Example
Given the following binary tree:

   1
 /   \
2     3
 \
  5

All root-to-leaf paths are:

[
  "1->2->5",
  "1->3"
]

Review

My Original implementation is using recursive in the methods. It needs more time since every node goes through all children’s paths. The review version is using a helper method and pass parent path to children.

Solution

Java (review version)

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param root the root of the binary tree
     * @return all root-to-leaf paths
     */
    public List<String> binaryTreePaths(TreeNode root) {
        // Write your code here
        List<String> paths = new ArrayList<String>();
        helper(root, "", paths);
        return paths;
    }
    
    private void helper(TreeNode node, String parentPath, List<String> paths) {
        if (node == null) {
            return;
        }
        String path = parentPath + node.val;
        if (node.left == null && node.right == null) {
            paths.add(path);
        }
        helper(node.left, path + "->", paths);
        helper(node.right, path + "->", paths);
        return;
    }
}

Java

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param root the root of the binary tree
     * @return all root-to-leaf paths
     */
    public List<String> binaryTreePaths(TreeNode root) {
        // Write your code here
        List<String> paths = new ArrayList<String>();
        if (root == null) {
            return paths;
        }
        if (root.left == null && root.right == null) {
            paths.add(String.valueOf(root.val));
        }
        List<String> lpaths = binaryTreePaths(root.left);
        for (String path : lpaths) {
            paths.add(root.val + "->" + path);
        }
        List<String> rpaths = binaryTreePaths(root.right);
        for (String path : rpaths) {
            paths.add(root.val + "->" + path);
        }
        return paths;
    }
}