LeetCode - MinStack

최소값을 상수시간에 리턴하는 stack만들기. stack을 만들면서 간단히 해당 element의 상태일때 최소값을 기억해놨다가 리턴해주면 된다.

 

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

Implement the MinStack class:

• MinStack() initializes the stack object.
• void push(val) pushes the element val onto the stack.
• void pop() removes the element on the top of the stack.
• int top() gets the top element of the stack.
• int getMin() retrieves the minimum element in the stack.

 

Example 1:

Input ["MinStack","push","push","push","getMin","pop","top","getMin"] [[],[-2],[0],[-3],[],[],[],[]] Output [null,null,null,null,-3,null,0,-2] Explanation MinStack minStack = new MinStack(); minStack.push(-2); minStack.push(0); minStack.push(-3); minStack.getMin(); // return -3 minStack.pop(); minStack.top(); // return 0 minStack.getMin(); // return -2

 

Constraints:

• -231 <= val <= 231 - 1
• Methods pop, top and getMin operations will always be called on non-empty stacks.
• At most 3 * 104 calls will be made to push, pop, top, and getMin.

class MinStack {

    /** initialize your data structure here. */
    int[] array = new int[30000];
    int[] minArray = new int[30000];

    int top = 0;
    int bottom = 0;

    int minVal = Integer.MAX_VALUE;

    void push(int val) {
        // min값 두개중에 하나에 넣는다
        // 두개중에 하나에 넣는게 아니라 둘중에 큰거를 넣는다
        // 그래야 제일 작은게 2개가 남는다
        array[top] = val;
        if (minVal > val) {
            minVal = val;
        }
        minArray[top] = minVal;
        top++;
    }

    void pop() {
        array[top - 1] = 0;
        minArray[top - 1] = 0;
        top--;
        if (top - 1 < 0) {
            minVal = Integer.MAX_VALUE;
        } else {
            minVal = minArray[top - 1];
        }
    }

    int top() {
        return array[top - 1];
    }

    int getMin() {
        return minArray[top - 1];
    }
}

/**
 * Your MinStack object will be instantiated and called as such:
 * MinStack obj = new MinStack();
 * obj.push(val);
 * obj.pop();
 * int param_3 = obj.top();
 * int param_4 = obj.getMin();
 */