Leetcode-Maximum Depth of Binary Tree

Given a binary tree, find its maximum depth.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

思路：在求樹的最小深度算法上稍作改動就可實現找到最大深度。

AC代碼：

import java.util.ArrayList;

public class Solution {
    public int maxDepth(TreeNode root) {
        if( root == null ) return 0;
		ArrayList<TreeNode> lastLevel = new ArrayList<TreeNode>();
		int count = 1;
		lastLevel.add(root);
		while( !lastLevel.isEmpty() ){
			ArrayList<TreeNode> nextLevel = new ArrayList<TreeNode>();
			for( TreeNode treenode:lastLevel ){
				if( treenode.left != null ) nextLevel.add(treenode.left);
				if( treenode.right != null ) nextLevel.add(treenode.right);
			}
			if( nextLevel.isEmpty() ) break;
			count++;
			lastLevel = new ArrayList<TreeNode>(nextLevel);
		}
		return count;
    }
}