Binary Tree Paths.java

/*
    Given a binary tree, return all root-to-leaf paths.
    
    Link: https://leetcode.com/problems/binary-tree-paths/

    Example: For example, given the following binary tree:
		   1
		 /   \
		2     3
		 \
		  5
		All root-to-leaf paths are:

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

    Solution: None
    
    Source: None
*/

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