题目描述：

给一棵二叉树，找出从根节点到叶子节点的所有路径。

样例：

给出下面这棵二叉树：

   1
 /   \
2     3
 \
  5
所有根到叶子的路径为：

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

代码实现：

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param root the root of the binary tree
     * @return all root-to-leaf paths
     */
    public List<String> binaryTreePaths(TreeNode root) {
        List<String> paths = new ArrayList<>();
        if (root == null) {
            return paths;
        }
        //root is a leaf
        if (root.left == null && root.right == null) {
            paths.add(root.val + "");
        }
        
        List<String> leftPath = binaryTreePaths(root.left);
        List<String> rightPath = binaryTreePaths(root.right);
        for (String path : leftPath) {
            paths.add(root.val + "->" + path);
        }
        for (String path : rightPath) {
            paths.add(root.val + "->" + path);
        }
        return paths;
    }
}