Implement a stack with min() function, which will return the smallest number in the stack.
It should support push, pop and min operation all in O(1) cost.
Notice
min operation will never be called if there is no number in the stack.
min operation will never be called if there is no number in the stack.
分析
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
|
public class MinStack {
private Stack<Integer> stack = new Stack<Integer>();
private Stack<Integer> stack_min = new Stack<Integer>();
public MinStack() {
// do initialize if necessary
}
public void push(int number) {
// write your code here
stack.push(number);
if(!stack_min.empty() && stack_min.peek() < number){
stack_min.push(stack_min.peek());
}
else{
stack_min.push(number);
}
}
public int pop() {
// write your code here
int top = -1;
if(!stack.empty()){
top = stack.peek();
stack.pop();
stack_min.pop();
}
return top;
}
public int min() {
// write your code here
return stack_min.empty() ? -1 : stack_min.peek();
}
} |