LeetCode: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.

思路：
利用前序遍歷的方法，將每個節點的值更新爲root到該節點的sum。遍歷完之後，所有的葉節點的val保存的便是root to leaf的number了。然後再用前序遍歷的方法，找到所有葉節點val用list保存起來，最後求和就得到結果了。

/**
 * 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) {
        List<Integer> result=new ArrayList<Integer>();
        if(root==null)return 0;
        preorder(root,0);
        int sum=0;
        sum(root,result);
        for(int e:result){
            sum+=e;
        }
        return sum;
    }
    public void sum(TreeNode node,List<Integer>result){//遍歷葉節點,同時這也是尋找葉節點的方法 
        if(node==null)return;
        if(node.left==null&&node.right==null) result.add(node.val);
        sum(node.left,result);
        sum(node.right,result);
    }
    public void preorder(TreeNode node,int sum){//更新每個節點的值 
          if(node==null) return;
          node.val=sum*10+node.val;
          preorder(node.left,node.val);
          preorder(node.right,node.val);

    }
}