LeetCode 116: Populating Next Right Pointers in Each Node
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Difficulty: 3
Frequency: 3
Problem:
Given a binary tree
struct TreeLinkNode { TreeLinkNode *left; TreeLinkNode *right; TreeLinkNode *next; }
Populate each next pointer to point to its next right node. If there is no next right node, the next pointer should be set to NULL
.
Initially, all next pointers are set to NULL
.
Note:
- You may only use constant extra space.
- You may assume that it is a perfect binary tree (ie, all leaves are at the same level, and every parent has two children).
For example,
Given the following perfect binary tree,
1 / \ 2 3 / \ / \ 4 5 6 7
After calling your function, the tree should look like:
1 -> NULL / \ 2 -> 3 -> NULL / \ / \ 4->5->6->7 -> NULL
Solution:
/** * Definition for binary tree with next pointer. * struct TreeLinkNode { * int val; * TreeLinkNode *left, *right, *next; * TreeLinkNode(int x) : val(x), left(NULL), right(NULL), next(NULL) {} * }; */class Solution {public: void connect(TreeLinkNode *root) { // Start typing your C/C++ solution below // DO NOT write int main() function connectPerfectRecursive(root); } void connectPerfectRecursive(TreeLinkNode * root) { if (root==NULL) return; if (root->left==NULL) { root->next = NULL; return; } connectPerfectRecursive(root->left); connectPerfectRecursive(root->right); TreeLinkNode *left = root->left, *right = root->right; while(left!=NULL) { left->next = right; left = left->right; right = right->left; } }};
Notes:
Although my code passed all tests, I still don't understand the requirement: you may only useconstant extra space. In this case, I think tree traverse is necessary. Tree traverse is either recursive or using a stack. Neither of them meets the constant extra space requirement. I will consult others' codes later.
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