#include<bits/stdc++.h>
using namespace std;
int main(){
    long N,i,stack_id,num;
    string code;
    const string push="PUSH";
    vector< stack<int> > stacks;


    cin>>N;

    for(i=0;i<N;i++){
        cin>>code;

        if(code==push){
            cin>>stack_id>>num;

            if(stack_id>stacks.size()){
                stacks.push_back( stack<int>() );
            }
            stacks[stack_id-1].push(num);

        }
        else{
            cin>>stack_id;

            cout<<stacks[stack_id-1].top()<<"\n";
            stacks[stack_id-1].pop();
        }
    }

    return 0;
}



I get correct output on the sample code though.

Try test:
1
push 900 1

Btw, have you seen memory limit for this task?
You need to be able to execute 100,000 pushes of 4-bytes integers (in one stack and in random stacks) - 400 Kb of data + empty program requires ~200K.

Your implementation should fail (memory limit) during vector resize.