116-Populating Next Right Pointers in Each Node

Description

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).

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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

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問題描述

二叉樹中節點的結構如下

struct TreeLinkNode {
  TreeLinkNode *left;
  TreeLinkNode *right;
  TreeLinkNode *next;
}

將next指向右邊的節點。如果某節點無右邊節點， next置為null

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問題分析

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解法1

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;
        }
    }
}

解法2

public class Solution {
    public void connect(TreeLinkNode root) {
        if(root == null) return;

        List<TreeLinkNode> res = new ArrayList<TreeLinkNode>();
        connectFunc(root, res, 0);
    }
    public void connectFunc(TreeLinkNode root, List<TreeLinkNode> res, int height){
        if(root == null) return;

        if(height == res.size()) res.add(root);
        else{
            res.get(height).next = root;
            res.set(height,root);
        }

        connectFunc(root.left, res, height + 1);
        connectFunc(root.right, res, height + 1);
    }
}

解法3

public class Solution {
    public void connect(TreeLinkNode root) {
        if(root == null)    return;
        if(root.left != null){
            root.left.next = root.right;
            if(root.next != null)   root.right.next = root.next.left;
        }

        connect(root.left);
        connect(root.right);
    }
}