111. Minimum Depth of Binary Tree

📋 Đề Bài

Given a binary tree, find its minimum depth.

The minimum depth is the number of nodes along the shortest path from the root node down to the nearest leaf node.

Note: A leaf is a node with no children.

Example 1:

Input: root = [3,9,20,null,null,15,7]
Output: 2

Example 2:

Input: root = [2,null,3,null,4,null,5,null,6]
Output: 5

Constraints:

The number of nodes in the tree is in the range [0, 10⁵].
-1000 <= Node.val <= 1000

🧠 Thuật Toán & Kỹ Thuật

Tree Traversal (Duyệt cây)

⏱️ Thời gian O(n)

💾 Không gian O(1)

💻 Lời Giải

C++ 0111-minimum-depth-of-binary-tree.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 minDepth(TreeNode* root) {
        if (!root) {
            return 0;
        }
        else if (root->left == nullptr and root->right == nullptr) {
            return 1;
        }
        else if (root->left == nullptr) {
            return 1 + minDepth(root->right);
        }
        else if (root->right == nullptr) {
            return 1 + minDepth(root->left);
        }
        return min(minDepth(root->left), minDepth(root->right)) + 1;
    }
};