LeetCode_Tree_Populating Next Right Pointers in Each Node I II

Populating Next Right Pointers in Each Node I

一、题目说明

如图所说，将二叉树中每一层作指向。

二、解题思路

按层级遍历树的节点，使得横向前节点指向后节点。然后再从下一层第一个节点开始，直到每一层都遍历结束。

三、java代码

/**
 * 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) {
            //记录level_start的首个节点，以便于遍历下一层
            TreeLinkNode cur = level_start;
            while(level_start != null) {
                if(level_start.left != null) {
                    level_start.left.next = level_start.right;
                }

                if(level_start.right != null && level_start.next != null) {
                    level_start.right.next = level_start.next.left;
                }
                level_start = level_start.next;
            }
            level_start = cur.left; 
        }
    }
}

Populating Next Right Pointers in Each Node II

一、题目说明

如 I 中所说，II 中说树为任意二叉树。

二、解题思路

思路跟上一道题一样，按层级遍历树的节点，使得横向前节点指向后节点。然后再从下一层第一个节点开始，直到每一层都遍历结束。

但是不一样的是每一层的开始节点不保证是level_start.left；下一层的第一个节点具有不确定性，所以需要引入dummy节点。

三、java代码

/**
 * 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) {
            //下一层的首个节点
            TreeLinkNode dummy = new TreeLinkNode(0);
            TreeLinkNode cur = dummy;
            while(level_start != null) {
                if(level_start.left != null) {
                    cur.next = level_start.left;
                    cur = cur.next;
                }
                if(level_start.right != null) {
                    cur.next = level_start.right;
                    cur = cur.next;
                }
                level_start = level_start.next;
            }
            level_start = dummy.next; 
        }

    }
}