[LeetCode]Sum Root to Leaf Numbers

Question

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

.

本题难度Medium。

DFS

【复杂度】

时间 O(N) 空间 O(1)

【思路】

这道题并不难，从上而下地进行递推累加，然后返回其累加值。

【注意】

叶子节点是其左右子树都为null。只有一个子树为null的不是叶子节点！

【代码】

/**
* 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) {
//invariant
return helper(root,0);
}
private int helper(TreeNode root,int pre){
//base case
if(root==null)
return 0;
if(root.left==null&&root.right==null)
return pre*10+root.val;

return helper(root.left,pre*10+root.val)+helper(root.right,pre*10+root.val);
}
}