lintcode--二叉树的所有路径

给一棵二叉树，找出从根节点到叶子节点的所有路径。

您在真实的面试中是否遇到过这个题？

Yes

样例

给出下面这棵二叉树：

所有根到叶子的路径为：

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

/**
 * 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;
 *     }
 * }
 */
//深度优先 可以转换成先序遍历：根左右，根结点遍历以后，遍历两个子树，是叶子结点的时候保存路径
// version 1: Divide Conquer分治法
public class Solution {
    public List<String> binaryTreePaths(TreeNode root) {
        List<String> paths = new ArrayList<>();
        if (root == null) {
            return paths;
        }
        List<String> leftPaths = binaryTreePaths(root.left);
        List<String> rightPaths = binaryTreePaths(root.right);
        
        for (String path : leftPaths) {
            paths.add(root.val + "->" + path);
        }
        for (String path : rightPaths) {
            paths.add(root.val + "->" + path);
        }
        
        // 如果root是叶子结点
        if (paths.size() == 0) {
            paths.add("" + root.val);
        }
        
        return paths;
    }
}