The celebrity problem

/*

Logic is :there can be only on celebrity possible,because for being another celebrity,first celebrity must know the other celebrity
and if first celebrity knows the second celebrity,then first celebrity can't be celebrity because he knows someone else.
so using stack,i found that one probable celebrity.
*/
class Solution:
    def celebrity(self, mat):
        st=[i for i in range(len(mat))]
        while len(st)>1:
            a,b=st.pop(),st.pop()
            if mat[a][b]==1:
                st.append(b)
            else:
                st.append(a)
        k=st[-1]
        for i in range(len(mat)):
            if k==i:
                continue;
            if mat[i][k]==0 or mat[k][i]==1:
                return -1
        return k