func minDepth(root *TreeNode) int {
	if root == nil {
		return 0
	}
	minDepth := math.MaxInt64
	var dfs func(node *TreeNode, depth int)
	dfs = func(node *TreeNode, depth int) {
		if node == nil {
			return
		}
		depth ++
		if node.Left == nil && node.Right == nil {
			if depth < minDepth {
				minDepth = depth 
			}
		}
		dfs(node.Left, depth)
		dfs(node.Right, depth)
	}
	dfs(root, 0)
	return minDepth
}