[LeetCode] 257. Binary Tree Paths

Given a binary tree, return all root-to-leaf paths.
For example, given the following binary tree:
   1
 /   \
2     3
 \
  5
All root-to-leaf paths are:
["1->2->5", "1->3"]

Thought process:
DFS.

Solution:

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<String> binaryTreePaths(TreeNode root) {
        List<String> paths = new ArrayList<>();
        if (root == null) {
            return paths;
        }
        String path = Integer.toString(root.val);
        binaryTreePaths(root, path, paths);
        return paths;
    }
    
    private void binaryTreePaths(TreeNode node, String path, List<String> paths) {
        if (node.left == null && node.right == null) {
            paths.add(path);
            return;
        }
        if (node.left != null) {
            binaryTreePaths(node.left, path + "->" + node.left.val, paths);
        }
        if (node.right != null) {
            binaryTreePaths(node.right, path + "->" + node.right.val, paths);
        }
    }
}

Time complexity: O(n).
Location: Philadelphia, PA, USA

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