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

 1 /**
 2  * Definition of TreeNode:
 3  * public class TreeNode {
 4  *     public int val;
 5  *     public TreeNode left, right;
 6  *     public TreeNode(int val) {
 7  *         this.val = val;
 8  *         this.left = this.right = null;
 9  *     }
 
10  * }
11  */
12 
13 public class Solution {
14     /**
15      * @param root: the root of the binary tree
16      * @return: all root-to-leaf paths
17      */
18     public List<String> binaryTreePaths(TreeNode root) {
19         // write your code here
20         List<String> list = new ArrayList<>();
 
21         StringBuilder sb = new StringBuilder();
22         traversal(list, root, sb);
23         return list;
24     }
25 
26     private void traversal(List<String> list, TreeNode root, StringBuilder sb) {
27         if (root == null) {
28             return;
29         }
30         if (root.left == null
 
 && root.right == null) {
31             sb.append(root.val);
32             list.add(new String(sb));
33             return;
34         }
35         int len = sb.length();
36         if (root.left != null) {
37             sb.append(root.val + "->");
38             traversal(list, root.left, sb);
39             sb.delete(len, sb.length());
40         }
41         if (root.right != null) {
42             sb.append(root.val + "->");
43             traversal(list, root.right, sb);
44             sb.delete(len, sb.length());
45         }
46     }
47 }