257. Binary Tree Paths

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"]

/**
 * 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> res = new ArrayList<>();
        String s="";
        if(root==null) return res;
        preOrder(root, res, s);
        return res;
    }
    public void preOrder(TreeNode root, List<String> res, String s){
        s=s+root.val+"->";
        //only when both children are null, means end of path, one child is null, means a turn node, so keep going down
        if(root.left==null && root.right==null){
            res.add(s.substring(0,s.length()-2));
            return;
        }
        if(root.left!=null)
            preOrder(root.left, res, s);
        if(root.right!=null)
            preOrder(root.right, res, s);
        
    }
}