[LeetCode]Sum Root to Leaf Numbers
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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); }}
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