LC 二叉树的最大深度

https://leetcode-cn.com/leetbook/read/top-interview-questions-easy/xnd69e/

Recursion

/**
 * 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) {
        return root == null ? 0: Math.max(maxDepth(root.left), maxDepth(root.right)) + 1;
    }
}

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
 #include <algorithm>    // std::max
class Solution {
public:
    int maxDepth(TreeNode* root) {
        return root == nullptr ? 0 : std::max(maxDepth(root->left),maxDepth(root->right)) + 1;
    }
};

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution(object):
    def maxDepth(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        return 0 if root == None else max(self.maxDepth(root.left),self.maxDepth(root.right))+1

BFS

/**
 * 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;
        //创建一个队列
        Deque<TreeNode> deque = new LinkedList<>();
        deque.push(root);
        int count = 0;
        while (!deque.isEmpty()) {
            //每一层的个数
            int size = deque.size();
            while (size-- > 0) {
                TreeNode cur = deque.pop();
                if (cur.left != null)
                    deque.addLast(cur.left);
                if (cur.right != null)
                    deque.addLast(cur.right);
            }
            count++;
        }
        return count;
    }

}

DPS

/**
 * 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;
        //stack记录的是节点，而level中的元素和stack中的元素
        //是同时入栈同时出栈，并且level记录的是节点在第几层
        Stack<TreeNode> stack = new Stack<>();
        Stack<Integer> level = new Stack<>();
        stack.push(root);
        level.push(1);
        int max = 0;
        while (!stack.isEmpty()) {
            //stack中的元素和level中的元素同时出栈
            TreeNode node = stack.pop();
            int temp = level.pop();
            max = Math.max(temp, max);
            if (node.left != null) {
                //同时入栈
                stack.push(node.left);
                level.push(temp + 1);
            }
            if (node.right != null) {
                //同时入栈
                stack.push(node.right);
                level.push(temp + 1);
            }
        }
        return max;
    }

}