Binary Tree Paths

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

For example, given the following binary tree:

  1.    1
  2.  /   \
  3. 2     3
  4.  \
  5.   5

All root-to-leaf paths are:

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

Solution:

  1. /**
  2.  * Definition for a binary tree node.
  3.  * public class TreeNode {
  4.  *     int val;
  5.  *     TreeNode left;
  6.  *     TreeNode right;
  7.  *     TreeNode(int x) { val = x; }
  8.  * }
  9.  */
  10. public class Solution {
  11.   public List<String> binaryTreePaths(TreeNode root) {
  12.     List<String> res = new ArrayList<String>();
  13.     helper(root, new ArrayList<Integer>(), res);
  14.     return res;
  15.   }
  17.   void helper(TreeNode root, List<Integer> sol, List<String> res) {
  18.     if (root == null) return;
  20.     sol.add(root.val);
  22.     if (root.left == null && root.right == null) {
  23.       res.add(convert(sol));
  24.     } else {
  25.       helper(root.left, sol, res);
  26.       helper(root.right, sol, res);
  27.     }
  29.     sol.remove(sol.size() - 1);
  30.   }
  32.   String convert(List<Integer> list) {
  33.     StringBuilder sb = new StringBuilder();
  35.     for (int i = 0; i < list.size(); i++) {
  36.       if (i > 0) sb.append("->");
  37.       sb.append(list.get(i));
  38.     }
  40.     return sb.toString();
  41.   }
  42. }