LeetCode刷题笔录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

.

还是不大习惯递归，总是本能地去用非递归的方法。不过这题还是递归比较简单好写。把到当前节点为止的sum作为参数进行传递。遇到leaf的话就把sum*10+node.val返回。sum*10就是这题的要点了。

这里有一个比较详尽的想法。

/**
* Definition for binary tree
* 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;
return sumNumbers(root, 0);
}

public int sumNumbers(TreeNode node, int sum){
if(node == null)
return 0;
if(node.left == null && node.right == null)
return sum * 10 + node.val;
return sumNumbers(node.left, sum * 10 + node.val) + sumNumbers(node.right, sum * 10 + node.val);
}

}