111. Minimum Depth of Binary Tree

// Approach: DFS recursion. A node with one null child is not a leaf, so
// recurse only on the non-null side in that case.
// Time: O(n) Space: O(h)

public class TreeNode
{
    public int val;
    public TreeNode left;
    public TreeNode right;
    public TreeNode(int val = 0, TreeNode left = null, TreeNode right = null)
    {
        this.val = val;
        this.left = left;
        this.right = right;
    }
}

public class Solution
{
    public int MinDepth(TreeNode root)
    {
        if (root == null)
            return 0;

        if (root.left == null)
            return MinDepth(root.right) + 1;

        if (root.right == null)
            return MinDepth(root.left) + 1;

        return Math.Min(MinDepth(root.left), MinDepth(root.right)) + 1;
    }
}