《leetCode》：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.

思路

第一步：找出所有的从root到叶子节点的路径
第二步：求和

实现代码如下：

/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */
public class Solution {
    private List<List<Integer>> res=new ArrayList<List<Integer>>();//用来保存所有的路径
    public int sumNumbers(TreeNode root) {
        if(root==null){
            return 0;
        }
        //第一步：找出所有的路径
        sumNumbersHelper(root,new ArrayList<Integer>());
        //求和
        return sumAllList(res);
    }

    private int sumAllList(List<List<Integer>> listList) {
        if(listList==null){
            return 0;
        }
        int sum=0;
        int listSum=0;
        for(List<Integer> temp:listList){
            listSum=0;//注意：要在这里初始化为零
            for(Integer num:temp){
                listSum=listSum*10+num;
            }
            sum+=listSum;
        }
        return sum;
    }

    private void sumNumbersHelper(TreeNode root,List<Integer> list) {
        if(root==null){
            return;
        }
        list.add(root.val);
        if(root.left==null&&root.right==null){//到达叶子节点，保存此条路径 
            res.add(list);
            return ;
        }
        else{
            sumNumbersHelper(root.left,new ArrayList<Integer>(list));
            sumNumbersHelper(root.right,new ArrayList<Integer>(list));
        }
    }
}

小结

原以为这道题自己做不出来的，写了写着思路就开了，太开兴了，哈哈