111. Minimum Depth of Binary Tree

Description of the Problem

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, 10^5].
-1000 <= Node.val <= 1000

Solution

Code (C++)

class Solution {
public:
    int minDepth(TreeNode* root) {
        if (root == nullptr) return 0;
        int left = minDepth(root->left);
        int right = minDepth(root->right);
        return left == 0 ^ right == 0 ? 
            1 + std::max(left, right) :
            1 + std::min(left, right);
    }
};

Complexity

n is the number of nodes in the tree
h is the height of the tree

Time complexity:

\( T(n) = O(n) \)

Auxiliary Space:

\( S(n) = O(h) \)

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