leetcode题解（四十二）：105. Construct Binary Tree from Preorder and Inorder Traversal

给出前序和中序遍历，构造二叉树

通过前序遍历得到根节点，中序遍历得到左右子树，递归下去就可以了

public TreeNode buildTree(int[] preorder, int[] inorder) {
    return helper(0, 0, inorder.length - 1, preorder, inorder);
}

public TreeNode helper(int preStart, int inStart, int inEnd, int[] preorder, int[] inorder) {
    if (preStart > preorder.length - 1 || inStart > inEnd) {
        return null;
    }
    //root就是preorder的prestart指针所指的数字
    TreeNode root = new TreeNode(preorder[preStart]);
    int inIndex = 0; // Index of current root in inorder
    for (int i = inStart; i <= inEnd; i++) {
    	//	更新inorder的index
        if (inorder[i] == root.val) {
            inIndex = i;
        }
    }
    root.left = helper(preStart + 1, inStart, inIndex - 1, preorder, inorder);
    root.right = helper(preStart + inIndex - inStart + 1, inIndex + 1, inEnd, preorder, inorder);
    return root;
}