原创转载请注明出处：http://agilestyle.iteye.com/blog/2360962
Solution
1. Using 2 stacks
1.1. One stack to accept incoming values in FILO manner
1.2. The other stack to reverse the values in first stack, so LIFO+LIFO becomes FIFO for value retrieval
2. An example
2.1. Incoming order: (1,2,3)
2.2. Stack 1 storage order(top to bottom): (3,2,1)
2.3. Stack 2 after pushing and popping all values in Stack 1:(top to bottom): (1,2,3)
3. One important thing
The reverse of all values in Stack 1 to stack 2 happens only stack 2 is empty!!!
package org.fool.java.test;
import java.util.Stack;
public class MyStackQueueTest {
public static void main(String[] args) throws Exception {
MyStackQueue<Integer> q = new MyStackQueue<>();
q.enQueue(1);
q.enQueue(2);
q.enQueue(3);
q.enQueue(4);
System.out.println(q.deQueue());
System.out.println(q.deQueue());
System.out.println(q.deQueue());
System.out.println(q.deQueue());
q.enQueue(5);
q.enQueue(6);
q.enQueue(7);
System.out.println(q.deQueue());
System.out.println(q.deQueue());
System.out.println(q.deQueue());
}
}
class MyStackQueue<T> {
Stack<T> s1 = new Stack<>();
Stack<T> s2 = new Stack<>();
public void enQueue(T k) {
s1.push(k);
}
public T deQueue() throws Exception {
if(s2.isEmpty() && s1.isEmpty()) { // if no elements at all
throw new Exception("no elements!");
} else if(s2.isEmpty()) { // get all elements in s1 and save to s2
while (!s1.isEmpty()) {
s2.push(s1.pop());
}
}
return s2.pop();
}
}
Console Output
Reference