public int minDepth(TreeNode root) {
        if (root == null) return 0;

        int left = minDepth(root.left);
        int right = minDepth(root.right);

        if (left == 0 || right == 0)
            return Math.max(left, right) + 1;
        else
            return Math.min(left, right) + 1;
    }
Given Binary Tree, return minimum depth of Binary Tree.
src/problems/easy/MinimumDepthofBinaryTree.java