const minDepth = function(root) {
 if (root === null) return 0
 if (root.left === null && root.right === null) {
 return 1
 }
 if (root.left === null) {
 return 1 + minDepth(root.right)
 }
 if (root.right === null) {
 return 1 + minDepth(root.left)
 }
 return Math.min(minDepth(root.left), minDepth(root.right)) + 1
}