[leetcode] problem 106 – construct binary tree from inorder and postorder traversal

public class  {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode(int x) { val = x; }
}

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

private TreeNode helper(int[] inorder, int[] postorder, int inStart, int inEnd, int postStart, int postEnd) {
    if (inStart > inEnd || postStart > postEnd)
        return null;

    int i = inStart;

    for (; i <= inEnd; i++) {
        if (inorder[i] == postorder[postEnd])
            break;
    }

    TreeNode root = new TreeNode(postorder[postEnd]);
    root.left = helper(inorder, postorder, inStart, i - 1, postStart, postEnd - inEnd + i - 1);
    root.right = helper(inorder, postorder, i + 1, inEnd, postEnd - inEnd + i, postEnd - 1);

    return root;
}