全部根到葉子結點組組成的數字相加

原題

　　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.

題目大意

　　給定一個二叉樹，每一個結點的值是0-9，根到葉子組成一個數字，求全部的根到葉子組成的數字的和。

解題思路

　　採用回溯法。

代碼實現

算法實現類

public class Solution {

    private int result = 0; // 記錄總的結果
    private int num = 0; // 記根到葉子的數字

    public int sumNumbers(TreeNode root) {
        sum(root);
        return result;
    }

    private void sum(TreeNode root) {
        if (root != null) {
            num = num*10 + root.val;

            // 已經到了根結點了
            if (root.left == null && root.right == null) {
                result += num;
            }
            sum(root.left);
            sum(root.right);
            num /= 10;
        }
    }
}