【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.

【思路】

二叉树的穷尽搜索，递归法即可。

【Java代码】

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    int sum = 0;
    
    public int sumNumbers(TreeNode root) {
        if (root == null) return 0;
        
        run(root, 0);
        
        return sum;
    }
    
    public void run(TreeNode root, int num) {
        num = 10 * num + root.val;
        
        if (root.left == null && root.right == null) {
            sum += num;
        } 
        
        if (root.left != null) {
            run(root.left, num);
        }
        
        if (root.right != null) {
            run(root.right, num);
        }
    }
}

【LeetCode】Sum Root to Leaf Numbers 解题报告