LeetCode 662. Maximum Width of Binary Tree

原题链接在这里：https://leetcode.com/problems/maximum-width-of-binary-tree/

题目：

Given a binary tree, write a function to get the maximum width of the given tree. The width of a tree is the maximum width among all levels. The binary tree has the same structure as a full binary tree, but some nodes are null.

The width of one level is defined as the length between the end-nodes (the leftmost and right most non-null nodes in the level, where the null nodes between the end-nodes are also counted into the length calculation.

Example 1:

Input: 

           1
         /   
        3     2
       /        
      5   3     9 

Output: 4
Explanation: The maximum width existing in the third level with the length 4 (5,3,null,9).

Example 2:

Input: 

          1
         /  
        3    
       /        
      5   3     

Output: 2
Explanation: The maximum width existing in the third level with the length 2 (5,3).

Example 3:

Input: 

          1
         / 
        3   2 
       /        
      5      

Output: 2
Explanation: The maximum width existing in the second level with the length 2 (3,2).

Example 4:

Input: 

          1
         / 
        3   2
       /       
      5       9 
     /         
    6           7
Output: 8
Explanation:The maximum width existing in the fourth level with the length 8 (6,null,null,null,null,null,null,7).

Note: Answer will in the range of 32-bit signed integer.

题解：

把每个点笔记上id. cur.id = i, 那么cur.left.id = 2*i, cur.right.id = 2*i+1.

找到每一行开始点的id, start. 和最后点的id, end. 这一行的最大宽度就是end-start+1.

Time Complexity: O(n).

Space: O(n).

AC Java: 

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int widthOfBinaryTree(TreeNode root) {
        if(root == null){
            return 0;
        }
        
        LinkedList<Combo> que = new LinkedList<>();
        que.add(new Combo(root, 1));
        int res = 1;
        
        while(!que.isEmpty()){
            int size = que.size();
            int start = que.peek().id;
            int end = start;
            
            while(size-- > 0){
                Combo cur = que.poll();
                end = cur.id;
                if(cur.node.left != null){
                    que.add(new Combo(cur.node.left, cur.id * 2));
                }
                
                if(cur.node.right != null){
                    que.add(new Combo(cur.node.right, cur.id * 2 + 1));
                }
            }
            
            res = Math.max(res, end - start + 1);
        }
        
        return res;
    }
}

class Combo{
    TreeNode node;
    int id;
    public Combo(TreeNode node, int id){
        this.node = node;
        this.id = id;
    }
}

类似Binary Tree Level Order Traversal.