LeetCode-662.Maximum Width of Binary Tree

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).

每层宽度=最右节点编号-最左节点编号+1；二叉树的宽度值=max(每层宽度)

class Solution {
    int max;
    public int widthOfBinaryTree(TreeNode root) {
        max =0;
        List<Integer> list = new ArrayList<Integer>();
        help(root,0,1,list);
        return max;
    }
    private void help(TreeNode root,int level,int index,List<Integer> list){
        if(root==null){
            return;
        }
        if(level==list.size()){//每行的第一个节点加入index
            list.add(index);
        }
        int len = index+1-list.get(level);
        max = max>len?max:len;
        help(root.left,level+1,index*2,list);
        help(root.right,level+1,index*2+1,list);
    }
}