257. Binary Tree Paths | 二叉树的路径

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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思路：采用深度优先遍历。

/**
 * 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> list = new ArrayList<>();
		binaryTreePathsHelper(root, list, new String());
		return list;
	}

	public void binaryTreePathsHelper(TreeNode root, List<String> list, String string) {
		if (root == null) {
			return;
		}
		if (root.left == null && root.right == null) {
				string = string+ root.val;
			list.add(string);
			return;
		}

		binaryTreePathsHelper(root.left, list, string + root.val + "->");
		binaryTreePathsHelper(root.right, list, string + root.val + "->");
	}
}