[LeetCode][Java] Sum Root to Leaf Numbers

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

.

题意：

给定一棵二叉树，仅仅包含0-9的数字元素，每一条从根节点到叶子节点的路径都可以代表一个数.

举个例子一条从根节点到叶子节点的路径为1->2->3
代表数为

123

.

找出所有的从根到叶子节点路径中所表示的数的和。

比如给定：

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

.

算法分析：

通过递归，思路比较清晰，主要就是考虑递归条件和结束条件。直接看代码，比较好理解。

AC代码：

<span style="font-family:Microsoft YaHei;font-size:12px;">/**
* Definition for binary tree
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
//又是递归
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);
}
}</span>