【三次过】Lintcode 1353. 根节点到叶节点求和

给定仅包含来自0-9的数字的二叉树，每个根到叶路径可以表示数字。举个例子：root-to-leaf路径1-> 2-> 3，它代表数字123，找到所有根到叶的数的总和

样例

Example:

Input: [1,2,3]
    1
   / \
  2   3
Output: 25
Explanation:
The root-to-leaf path 1->2 represents the number 12.
The root-to-leaf path 1->3 represents the number 13.
Therefore, sum = 12 + 13 = 25.

Example 2:

Input: [4,9,0,5,1]
    4
   / \
  9   0
 / \
5   1
Output: 1026
Explanation:
The root-to-leaf path 4->9->5 represents the number 495.
The root-to-leaf path 4->9->1 represents the number 491.
The root-to-leaf path 4->0 represents the number 40.
Therefore, sum = 495 + 491 + 40 = 1026.

注意事项

叶节点是没有子节点的节点

---

解题思路：

DFS.

/**
 * 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;
 *     }
 * }
 */

public class Solution {
    /**
     * @param root: the root of the tree
     * @return: the total sum of all root-to-leaf numbers
     */
    public int sumNumbers(TreeNode root) {
        // write your code here
        return dfs(root, 0);
    }
    
    private int dfs(TreeNode root, int prev){
        if(root == null) 
            return 0;

        int sum = root.val + prev * 10;
        if(root.left == null && root.right == null) {
            return sum;
        }

        return dfs(root.left, sum) + dfs(root.right, sum);
    }
}