LeetCode 129. Sum Root to Leaf Numbers

原题链接在这里：https://leetcode.com/problems/sum-root-to-leaf-numbers/

题目：

Given a binary tree containing digits from 0-9 only, each root-to-leaf path could represent a number.

An example is the root-to-leaf path 1->2->3 which represents the number 123.

Find the total sum of all root-to-leaf numbers.

For example,

    1
   / 
  2   3

The root-to-leaf path 1->2 represents the number 12.
The root-to-leaf path 1->3 represents the number 13.

Return the sum = 12 + 13 = 25.

题解：

Top-down dfs到了叶子节点加值.

Time Complexity: O(n).

Space: O(h), h是树的高度.

AC Java:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int sumNumbers(TreeNode root) {
        if(root == null){
            return 0;
        }
        
        int [] res = {0};
        dfs(root, 0, res);
        return res[0];
    }
    
    private void dfs(TreeNode root, int cur, int [] res){
        if(root == null){
            return;
        }
        
        cur = cur*10+root.val;
        
        if(root.left == null && root.right == null){
            res[0] += cur;
            return;
        }

        dfs(root.left, cur, res);
        dfs(root.right, cur, res);

    }
}

类似Path Sum II.