Minimum Depth of Binary Tree

Minimum Depth of Binary Tree

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

Solutions

🧠 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:
    //O(2^n) solution
    int minDepth(TreeNode* root)
    {
        if(!root)
            return 0;
        //if both leafs are null, we have the last leaf
        else if(!root->left && !root->right)
            return 1;

        //pre-order traversal
        return 1 + std::min(minDepth(root->left), minDepth(root->right), 
                            [](int a , int b)
                            {
                                if(!a) return false;
                                if(!b) return true;
                                return a < b;
                            });
    }
};