129. Sum Root to Leaf Numbers

Description

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,

tree

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.

Solution

DFS

注意递归的终止条件是leaf node，而非null。

/**
 * 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) {
        return sumNumbersRecur(root, 0);
    }
    
    public int sumNumbersRecur(TreeNode root, int sum) {
        if (root == null) {
            return 0;
        }
        
        if (root.left == null && root.right == null) {
            return sum * 10 + root.val;
        }
        
        return sumNumbersRecur(root.left, sum * 10 + root.val)
            + sumNumbersRecur(root.right, sum * 10 + root.val);
    }
}