Problem

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"]

Solution

public class Solution {    public List
    
      binaryTreePaths(TreeNode root) {        // Write your code here        List
     
       res = new ArrayList
      
       ();        if (root == null) {            return res;        }        findpaths(root, res, root.val + "");//这里+""巧妙地调用了String        return res;    }    public void findpaths(TreeNode root, List
       
         res, String cur) {               if (root.left != null) {           findpaths(root.left, res, cur + "->" + root.left.val);        }        if (root.right != null) {           findpaths(root.right, res, cur + "->" + root.right.val);        }        if (root.left == null && root.right == null) {           res.add(cur);           return;        }    }}
       
      
     
    

###Update 2018-9

class Solution {    public List
    
      binaryTreePaths(TreeNode root) {        List
     
       res = new ArrayList<>();        if (root == null) return res;        dfs(root, "", res);        return res;    }    private void dfs(TreeNode root, String str, List
      
        res) {        if (root.left == null && root.right == null) res.add(str+root.val);                if (root.left != null) dfs(root.left, str+root.val+"->", res);        if (root.right != null) dfs(root.right, str+root.val+"->", res);    }}