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Given a few numbers, how will you push them into a stack such that whenever I pop the top most element from that stack, I get the minimum number from the bunch. Also he wanted me to tell the pop push algorithm such that the order of insertion and popping should be O(1).

N*N matrix. contains only 0's and 1's.
every row is sorted in descending order.
find row containing maximum no of 1's. Efficient soln reqd.