116. Populating Next Right Pointers in Each Node

题目

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

分析

题目大意是要将每个结点和右边的结点用next连接起来

大致有三种情况

1.最右边的结点next直接是NULL

2.同一父母的左右孩子

3.不同父母的右孩子和左孩子

/**
* 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) {
if(!root) return;
while(root->left){
TreeLinkNode *p=root;
while(p){
p->left->next=p->right;
if(p->next) p->right->next=p->next->left;
p=p->next;
}
root=root->left;
}
}
};