104MaximumDepthofBinaryTree.org

[#C] 104 Maximum Depth of Binary Tree - LeetCode

题目

Given the root of a binary tree, return its maximum depth.

A binary tree’s maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

Example 1:

Input: root = [3,9,20,null,null,15,7]
Output: 3

Example 2:

Input: root = [1,null,2]
Output: 2

Example 3:

Input: root = []
Output: 0

Example 4:

Input: root = [0]
Output: 1

Constraints:

• The number of nodes in the tree is in the range [0, 104].
• -100 <= Node.val <= 100

递归法

Java

/**
 * 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));
    }
}

CPP

/**
 * 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) {}
 * };
 */
class Solution {
public:
    int maxDepth(TreeNode* root) {
        if (root == NULL) return 0;
        return 1 + max(maxDepth(root->left), maxDepth(root->right));
    }
};

广度优先

Java

/**
 * 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 bfs(root);
    }

    public int bfs(TreeNode node) {
        int level = 0;
        Queue<TreeNode> queue = new LinkedList();
        queue.offer(node);
        while(!queue.isEmpty()) {
            int size = queue.size();
            level++;
            for (int i = 0; i < size; i++) {
                TreeNode n = queue.poll();
                if (n.left != null) queue.offer(n.left);
                if (n.right != null) queue.offer(n.right);
            }
        }
        return level;
    }
}

CPP

/**
 * 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) {}
 * };
 */
class Solution {
public:
    int maxDepth(TreeNode* root) {
        int level = 0;
        if (root == nullptr) return level;
        queue<TreeNode*> q;
        q.push(root);
        while(!q.empty()) {
            level++;
            int size = q.size();
            for (int i = 0; i < size; ++i) {
                auto node = q.front();
                q.pop();
                if (node->left != nullptr) q.push(node->left);
                if (node->right != nullptr) q.push(node->right);
            }
        }
        return level;
    }
};

深度优先

Java

/**
 * 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 dfs(root);
    }

    public int dfs(TreeNode node) {
        int maxLevel = 0;
        Stack<TreeNode> stack = new Stack();
        Stack<Integer> depth = new Stack();
        stack.push(node);
        depth.push(1);
        while(!stack.isEmpty()) {
            TreeNode n = stack.pop();
            int level = depth.pop();
            maxLevel = Math.max(maxLevel, level);
            if (n.left != null) {
                stack.push(n.left);
                depth.push(level + 1);
            }
            if (n.right != null) {
                stack.push(n.right);
                depth.push(level + 1);
            }
           
        }
        return maxLevel;
    }
}

CPP

/**
 * 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) {}
 * };
 */
class Solution {
public:
    int maxDepth(TreeNode* root) {
        int maxLevel = 0;
        if (root == nullptr)
            return maxLevel;
        stack<TreeNode*> s;
        stack<int> level;
        s.push(root);
        level.push(1);
        while(!s.empty()) {
            TreeNode* node = s.top();
            s.pop();
            int l = level.top();
            level.pop();
            maxLevel = max(l, maxLevel);
        
            if (node->left != nullptr) {
                s.push(node->left);
                level.push(l + 1);
            }
        
            if (node->right != nullptr) {
                s.push(node->right);
                level.push(l + 1);
            }
        
        }
        return maxLevel;
    }

};