105. Construct Binary Tree from Preorder and Inorder Traversal.go

* Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func buildTree(preorder []int, inorder []int) *TreeNode {
    if len(preorder) == 0 || len(inorder) == 0 {
        return nil
    }
    root := new(TreeNode)
    root.Val = preorder[0]
    mid := -1
    for i, n := range inorder {
        if n == preorder[0] {
            mid = i
        }
    }
    root.Left = buildTree(preorder[1: mid + 1], inorder[:mid])
    root.Right = buildTree(preorder[mid + 1:], inorder[mid + 1:])
    return root
}