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”]

解题思路

先序遍历，递归的过程中保存路径，遇到叶子节点时将路径加入结果集。

实现代码

Java：

// Runtime: 3 ms
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    private List<String> paths = new ArrayList<String>();
    public List<String> binaryTreePaths(TreeNode root) {
        if (root != null) {
            traverse(root, String.valueOf(root.val));
        }

        return paths;
    }

    private void traverse(TreeNode root, String path) {
        if (root.left == null && root.right == null) {
            paths.add(path);
        }
        if (root.left != null) {
            traverse(root.left, path + "->" + root.left.val);
        }
        if (root.right != null) {
            traverse(root.right, path + "->" + root.right.val);
        }
    }
}