guass-seidel迭代法，matlab，实在看不出来有啥bug了，如何解决？

题目要求如下图

function [x,k] = GS(A,b)
n = size(A,2);
D =diag(diag(A));
L = -tril(A,-1);
U = -triu(A,1);
G = (D-L)\U;
f = (D-L)\b;
xs = A\b;
x = zeros(n,1);
k = 0;
while norm(x-xs,2) >= 1E-8
x = G*x+f;
k = k+1;
%fprintf('%3i %.3e \n',k,norm(x-xs,2));
end
end

我的判据错了吗，该怎么修改啊