129. Sum Root to Leaf Numbers

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

.

关键点：数值都在0-9之间，

左边的和+右边的和 不用考虑复杂

直接左右同时递归求和

public class Solution {
public int sumNumbers(TreeNode root) {
return preorder(root,0);
}
public int preorder(TreeNode root,int s){
if(root==null) return 0;
if(root.left==null && root.right==null) return s*10+root.val;
return preorder(root.left,s*10+root.val)+preorder(root.right,s*10+root.val);
}
}