117. Populating Next Right Pointers in Each Node II

题目：

Follow up for problem "Populating Next Right Pointers in Each Node".

What if the given tree could be any binary tree? Would your previous solution still work?

Note:

• You may only use constant extra space.

For example,
Given the following binary tree,

         1
       /  
      2    3
     /     
    4   5    7

After calling your function, the tree should look like:

         1 -> NULL
       /  
      2 -> 3 -> NULL
     /     
    4-> 5 -> 7 -> NULL

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 Tree Depth-first Search 

链接： http://leetcode.com/problems/populating-next-right-pointers-in-each-node-ii/

题解：

一样是DFS。跟上题不同在于，给定输入不是完全二叉树了，所以要加入一些条件来判断是否存在一个合理的next值。并且对左节点和右节点的有效性也要验证。最后要先递归连接右节点，再connect左节点。

Time Complexity - O(n)， Space Complexity - O(1)。

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

Update:

/**
 * 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) {
        if(root == null)
            return;
        TreeLinkNode p = root.next;
        
        while(p != null) {
            if(p.left != null) {
                p = p.left;
                break;
            } else if (p.right != null) {
                p = p.right;
                break;
            }
            p = p.next;    
        }
        
        if(root.right != null) 
            root.right.next = p;
        if(root.left != null) 
            root.left.next = (root.right != null) ? root.right : p;
        
        connect(root.right);
        connect(root.left);
    }
}

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二刷:

依然使用了递归，并没有做到constant space。留给三刷了。

Java:

Time Complexity - O(n)， Space Complexity - O(n)。

/**
 * 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) {
        if (root == null) return;
        TreeLinkNode nextNode = root.next;
        while (nextNode != null) {
            if (nextNode.left == null && nextNode.right == null) nextNode = nextNode.next;
            else break;
        }
        if (nextNode != null) nextNode = (nextNode.left != null) ? nextNode.left : nextNode.right;
        if (root.right != null) root.right.next = nextNode;
        if (root.left != null) root.left.next = (root.right != null) ? root.right : nextNode;
        connect(root.right);
        connect(root.left);
    }
}

iterative:

Level order traversal。主要就是类似二叉树层序遍历。这回把顶层看作一个linkedlist，我们只需要继续连接这linkedlist中每个节点的子节点们。当顶层遍历完毕以后，下一层正好也形成了一个新的类linkedlist。我们换到下一层以后继续遍历，直到最后。

Java:

Time Complexity - O(n)， Space Complexity - O(1)。

/**
 * 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 curLevel = new TreeLinkNode(-1);
        TreeLinkNode newLevel = curLevel;
        while (root != null) {
            if (root.left != null) {
                curLevel.next = root.left;
                curLevel = curLevel.next;
            }
            if (root.right != null) {
                curLevel.next = root.right;
                curLevel = curLevel.next;
            }
            root = root.next;
            if (root == null) {
                curLevel = newLevel;
                root = newLevel.next;
                newLevel.next = null;
            }
        }
    }
}

Update:

/**
 * 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) {
        if (root == null) return;
        TreeLinkNode curLevel = new TreeLinkNode(-1);
        TreeLinkNode newLevel = curLevel;
        while (root != null) {
            if (root.left != null) {
                curLevel.next = root.left;
                curLevel = curLevel.next;
            }
            if (root.right != null) {
                curLevel.next = root.right;
                curLevel = curLevel.next;
            }
            root = root.next;
            if (root == null) {
                root = newLevel.next;
                newLevel.next = null;
                curLevel = newLevel;
            }
         }
        
    }
}

Reference:

http://www.cnblogs.com/springfor/p/3889327.html

https://leetcode.com/discuss/67291/java-solution-with-constant-space

https://leetcode.com/discuss/60795/o-1-space-o-n-time-java-solution

https://leetcode.com/discuss/24398/simple-solution-using-constant-space

https://leetcode.com/discuss/65526/ac-python-o-1-space-solution-12-lines-and-easy-to-understand

https://leetcode.com/discuss/3339/o-1-space-o-n-complexity-iterative-solution