minStack

Min Stack

 

用两个这样的栈可以实现一个再O（1）内获取最小元的队列。

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.

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static class MinStack {
        final static int SIZE = 100000;
        int stk[] = new int[SIZE];
        int top = 0;
        int min = Integer.MAX_VALUE;
        int nextMin[] = new int[SIZE];

        public void push(int x) {
            if (x <= min) {
                min = x;
                nextMin[top] = top;
            } else {
                if (top - 1 >= 0)
                    nextMin[top] = nextMin[top - 1];
            }
            stk[top++] = x;
        }

        public void pop() {


            --top;
            if (stk[top] == min) {
                if (top - 1 >= 0)
                    min = stk[nextMin[top - 1]];
            }
            if(top==0){
                min = Integer.MAX_VALUE;
            }
        }


        public int top() {
            if (top > 0) ;
            return stk[top - 1];
        }


        public int getMin() {
            return min;
        }
    }