代码随想录day16

一、二叉树的最大深度（LeetCode104）

递归

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public int maxDepth(TreeNode root) {
        if(root == null){
            return 0;
        }
        return 1 + Math.max(maxDepth(root.left),maxDepth(root.right));
    }
}

二、n叉树的最大深度（LeetCode559）

递归

/*
// Definition for a Node.
class Node {
    public int val;
    public List<Node> children;

    public Node() {}

    public Node(int _val) {
        val = _val;
    }

    public Node(int _val, List<Node> _children) {
        val = _val;
        children = _children;
    }
};
*/

class Solution {
    public int maxDepth(Node root) {
        if(root == null){
            return 0;
        }
        int deep = 0;
        if(root.children != null){
            for(Node tem:root.children){
                deep = Math.max(deep,maxDepth(tem));
            }
        }
        return 1 + deep;
    }
}

三、二叉树的最小深度（LeetCode111）

递归

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public int minDepth(TreeNode root) {
        if(root == null){
            return 0;
        }
        int leftheight = minDepth(root.left);
        int rightheight = minDepth(root.right);
        int result = 0;
        if(root.left == null && root.right != null){
            result = 1 + rightheight;
        }else if(root.left != null && root.right == null){
            result = 1 + leftheight;
        }else{
            result = 1 + Math.min(leftheight,rightheight);
        }
        return result;
    }
}

四、完全二叉树的节点个数（LeetCode222）

递归

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    /**
     * 针对完全二叉树的解法
     *
     * 满二叉树的结点数为：2^depth - 1
     */
    public int countNodes(TreeNode root) {
        if(root == null){
            return 0;
        }
        TreeNode left = root.left;
        TreeNode right = root.right;
        int leftdeep = 0;
        int rightdeep = 0;
        while(left != null){
            left = left.left;
            leftdeep++;
        }
        while(right != null){
            right = right.right;
            rightdeep++;
        }
        if(leftdeep == rightdeep){
            return (int)Math.pow(2,leftdeep+1)-1;//(2<<leftDepth)-1;注意(2<<1)相当于2^2，所以leftDepth初始为0
        }
        int leftnum = countNodes(root.left);
        int rightnum = countNodes(root.right);
        int res = leftnum + rightnum + 1;
        return res;
    }
}