Sum Root to Leaf Numbers
題意：
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.
解題思路：
使用遞迴去作，因為java無法 pass by reference，因此傳一個陣列下去，其程式碼如下：
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public int sumNumbers(TreeNode root) {
if (root == null) {
return 0;
}
int[] sum = {0};
int current = 0;
helper(root, current, sum);
return sum[0];
}
private void helper(TreeNode root, int current, int[] sum) {
if (root == null) {
return;
}
current = current * 10 + root.val;
if (root.left == null && root.right == null) {
sum[0] = sum[0] + current;
return;
}
helper(root.left, current, sum);
helper(root.right, current, sum);
}
}