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MinimumStack.java
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MinimumStack.java
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package stacks;
import java.io.*;
import java.util.*;
/**
* in constant time (not constant space)
* Because we are taking O(n) space complexity
*/
public class MinimumStack {
public static class MinStack {
Stack<Integer> allData;
Stack<Integer> minData;
public MinStack() {
allData = new Stack<>();
minData = new Stack<>();
}
int size() {
return allData.size();
}
void push(int val) {
allData.push(val);
if ( minData.size() == 0 || minData.peek() > val) {
minData.push(val);
}
}
int pop() {
if (allData.size() == 0) {
System.out.println("Stack underflow");
return -1;
}
int val = allData.pop();
if (minData.size() > 0 && val == minData.peek()) {
minData.pop();
}
return val;
}
int top() {
if (allData.size() == 0) {
System.out.println("Stack underflow");
return -1;
}
return allData.peek();
}
int min(){
if (minData.size() == 0) {
System.out.println("Stack underflow");
return -1;
}
return minData.peek();
}
}
public static void main(String[] args) throws Exception {
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
MinStack st = new MinStack();
String str = br.readLine();
while(str.equals("quit") == false){
if(str.startsWith("push")){
int val = Integer.parseInt(str.split(" ")[1]);
st.push(val);
} else if(str.startsWith("pop")){
int val = st.pop();
if(val != -1){
System.out.println(val);
}
} else if(str.startsWith("top")){
int val = st.top();
if(val != -1){
System.out.println(val);
}
} else if(str.startsWith("size")){
System.out.println(st.size());
} else if(str.startsWith("min")){
int val = st.min();
if(val != -1){
System.out.println(val);
}
}
str = br.readLine();
}
}
}