Leetcode Populating Next Right Pointers in Each Node

Populating Next Right Pointers in Each Node

Total Accepted: 6129 Total
Submissions: 17922My Submissions

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

直接上递归法

void connect(TreeLinkNode *root) 
	{
		if (!root || !root->left) return;
		root->left->next = root->right;
		con(root->left, root->right);
	}
	void con(TreeLinkNode *lTree, TreeLinkNode *rTree)
	{
		if (lTree->left) lTree->left->next = lTree->right;
		if (rTree->left) rTree->left->next = rTree->right;
		else return;
		lTree->right->next = rTree->left;
		con(lTree->left, lTree->right);
		con(lTree->right, rTree->left);
		con(rTree->left, rTree->right);
	}

下面参考leetcode论坛上的程序写了个程序

//新知识点：
//重点注意：利用新构造的数据结构
void connect(TreeLinkNode* root) 
{
	if (!root || !root->left || !root->right)
		return;
	TreeLinkNode* rightSibling;
	TreeLinkNode* p1 = root;
	while (p1) 
	{
		rightSibling = p1->next? p1->next->left:NULL;
		p1->left->next = p1->right;
		p1->right->next = rightSibling;
		p1 = p1->next;
	}
	connect(root->left);
}

有人会觉得使用递归并不符合常数空间的题意，那么可以改成非递归：

void connect(TreeLinkNode *root) 
	{
		TreeLinkNode *nextLev = root? root->left:NULL;
		while (nextLev)
		{
			for ( ; root; root = root->next) 
			{
				TreeLinkNode *rightSibling = root->next? root->next->left:NULL;
				root->left->next = root->right;
				root->right->next = rightSibling;
			}
			root = nextLev;
			nextLev = nextLev->left;
		}
	}

//2014-2-17 update
	void connect(TreeLinkNode *root) 
	{
		if (!root) return;
		TreeLinkNode *next_level = root->left;
		while (next_level)
		{
			root->left->next = root->right;
			TreeLinkNode *pre = root->right;
			root = root->next;
			while (root)
			{
				pre->next = root->left;
				pre = root->left->next = root->right;
				root = root->next;
			}
			root = next_level;
			next_level = root->left;
		}
	}

//2014-2-17 update
	void connect(TreeLinkNode *root) 
	{
		if (!root) return;
		while (root->left)
		{
			TreeLinkNode *next_level = root->left;
			root->left->next = root->right;
			TreeLinkNode *pre=root->right;			
			for (root=root->next; root; root = root->next)
			{
				pre->next = root->left;
				pre = root->left->next = root->right;
			}
			root = next_level;
		}
	}