LeetCode – Min Stack

转自：http://www.programcreek.com/2014/02/leetcode-min-stack-java/

Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.

push(x) -- Push element x onto stack.
pop() -- Removes the element on top of the stack.
top() -- Get the top element.
getMin() -- Retrieve the minimum element in the stack.

Thoughts

An array is a perfect fit for this problem. We can use a integer to track the top of the stack. You can use the Stack class from Java SDK, but I think a simple array is more efficient and more beautiful.

Java Solution

class MinStack { private int[] arr = new int[100]; private int index = -1;   public void push(int x) { if(index == arr.length - 1){ arr = Arrays.copyOf(arr, arr.length*2); } arr[++index] = x; }   public void pop() { if(index>-1){ if(index == arr.length/2 && arr.length > 100){ arr = Arrays.copyOf(arr, arr.length/2); } index--; } }   public int top() { if(index > -1){ return arr[index]; }else{ return 0; } }   public int getMin() { int min = Integer.MAX_VALUE; for(int i=0; i<=index; i++){ if(arr[i] < min) min = arr[i]; } return min; } }