Populating Next Right Pointers in Each Node

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.
• Recursive approach is fine, implicit stack space does not count as extra space for this problem.
• You may assume that it is a perfect binary tree (ie, all leaves are at the same level, and every parent has two children).

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

---

 

Approach #1: C++.

/**
 * 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 == NULL) return;
        root->next = NULL;
        queue<TreeLinkNode*> q;
        if (root->left) q.push(root->left);
        if (root->right) q.push(root->right);
        
        while (!q.empty()) {
            int size = q.size();
            while (size--) {
                TreeLinkNode* cur = q.front();
                q.pop();
                if (size) cur->next = q.front();
                else cur->next = NULL;
                if (cur->left) q.push(cur->left);
                if (cur->right) q.push(cur->right);
            }
            
        }
    }
};

　　

Approach #2: Java.

/**
 * Definition for binary tree with next pointer.
 * public class TreeLinkNode {
 *     int val;
 *     TreeLinkNode left, right, next;
 *     TreeLinkNode(int x) { val = x; }
 * }
 */
public class Solution {
    public void connect(TreeLinkNode root) {
        TreeLinkNode level_start = root;
        while (level_start != null) {
            TreeLinkNode cur = level_start;
            while (cur != null) {
                if (cur.left != null) cur.left.next = cur.right;
                if (cur.right != null && cur.next != null) cur.right.next = cur.next.left;
                cur = cur.next;
            }
            level_start = level_start.left;
        }
    }
}

　　

Approach #3: Python.

# Definition for binary tree with next pointer.
# class TreeLinkNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
#         self.next = None

class Solution:
    # @param root, a tree link node
    # @return nothing
    def connect(self, root):
        if not root: return
        while root.left:
            cur = root.left
            prev = None
            while root:
                if prev: prev.next = root.left
                root.left.next = root.right
                prev = root.right
                root = root.next
            root = cur