leetcode-binary-tree-paths

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题目大意

　　https://leetcode.com/problems/binary-tree-paths/

　　给你一棵二叉树，输出所有root节点到leaf节点的路径。

题目分析

　　比较简单的递归。

代码

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/**
 * 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> ans;
    
    private void helper(TreeNode r, String path, int idx) {
        if (r == null) {
            return;
        }
        if (!path.equals("")) {
            path += "->";
        }
        if (r.left == null && r.right == null) {
            ans.add(path + r.val);
            return;
        }
        helper(r.left, path  + r.val, idx + 1);
        helper(r.right, path + r.val, idx + 1);
    }
    
    public List<String> binaryTreePaths(TreeNode root) {
        ans = new ArrayList<String>();
        helper(root, "", 0);
        return ans;
    }
}