【LeetCode】104. Maximum Depth of Binary Tree 解題報告

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104. Maximum Depth of Binary Tree / Easy

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

Constraints:

  • The number of nodes in the tree is in the range [0, 10^4].
  • -100 <= Node.val <= 100

Solution: Recursive

思路

利用 DFS 尋訪，每向下 call 一次就增加紀錄的深度，走到葉節點的時候拿目前的深度與最深的比較，不斷更新即可。

效能

Complexity

  • Time Complexity: O(N)
  • Space Complexity: O(N), best case is O(logN)

LeetCode Result

Code

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class Solution {
public:
    int maxDepth(TreeNode* root) {
        dfs(root, 0);
        return maxD;
    }
    
    void dfs(TreeNode* root, int depth) {
        if(!root) {
            maxD = max(maxD, depth);
            return;
        }
        
        dfs(root->left, depth+1);
        dfs(root->right, depth+1);
    }
    
private:
    int maxD = 0;
};