Min depth of tree

Given a binary tree, find its minimum depth.

• Time: O(N)
• Space: O(1)

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;
}