代码随想录训练营二刷第十六天 | 104.二叉树的最大深度 559.n叉树的最大深度 111.二叉树的最小深度 222.完全二叉树的节点个数

一、104.二叉树的最大深度

题目链接：https://leetcode.cn/problems/maximum-depth-of-binary-tree/
思路：后序遍历，层序遍历

class Solution {public int maxDepth(TreeNode root) {if (root == null) return 0;int left = maxDepth(root.left);int right = maxDepth(root.right);return Math.max(left, right) + 1;}
}

二、559.n叉树的最大深度

题目链接：https://leetcode.cn/problems/maximum-depth-of-n-ary-tree/
思路：后序遍历加for

class Solution {public int maxDepth(Node root) {if (root == null) return 0;if (root.children == null) return 1;int max = Integer.MIN_VALUE;for (Node child : root.children) {max = Math.max(max, maxDepth(child));}return max == Integer.MIN_VALUE ? 1 : max + 1;}
}

三、111.二叉树的最小深度

题目链接：https://leetcode.cn/problems/minimum-depth-of-binary-tree/
思路：这题用层序遍历最快

public int minDepth(TreeNode root) {if (root == null) return 0;return deep(root);}int deep(TreeNode root) {if (root.left == null && root.right == null) return 1;int left = Integer.MAX_VALUE, right = Integer.MAX_VALUE;if (root.left != null) {left = minDepth(root.left);}if (root.right != null) {right = minDepth(root.right);}return Math.min(left, right) + 1;}

四、222.完全二叉树的节点个数

题目链接：https://leetcode.cn/problems/count-complete-tree-nodes/
思路：从完全二叉树结构出发。

public int countNodes(TreeNode root) {if (root == null) return 0;TreeNode left = root.left, right = root.right;int leftNum = 0, rightNum = 0;while (left != null) {leftNum++;left = left.left;}while (right != null) {rightNum++;right = right.right;}if (leftNum == rightNum) {return (2 << leftNum) - 1;}return countNodes(root.left) + countNodes(root.right) + 1;}