257. Binary Tree Paths java solutions

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"]

Credits:
Special thanks to @jianchao.li.fighter for adding this problem and creating all test cases.

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

用递归即可。