LeetCode: Min Stack 解题报告

Min Stack

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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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SOLUTION 1:

比较直观。用一个min stack专门存放最小值，如果有比它小 或是相等的（有多个平行的最小值都要单独存放，否则pop后会出问题），

则存放其到minstack.具体看代码：

class MinStack {
    Stack<Integer> elements = new Stack<Integer>();
    Stack<Integer> minStack = new Stack<Integer>();
    
    public void push(int x) {
        elements.push(x);
        if (minStack.isEmpty() || x <= minStack.peek()) {
            minStack.push(x);
        }
    }

    public void pop() {
        if (elements.isEmpty()) {
            return;
        }
        
        // 这个地方太蛋疼了，居然要用equals...
        if (elements.peek().equals(minStack.peek())) {
            minStack.pop();
        }
        elements.pop();
    }

    public int top() {
        return elements.peek();       
    }

    public int getMin() {
        return minStack.peek();
    }
}

 2014.1229 redo.

class MinStack {
    Stack<Integer> s = new Stack<Integer>();
    Stack<Integer> min = new Stack<Integer>();
    
    public void push(int x) {
        s.push(x);
        if (min.isEmpty() || x <= min.peek()) {
            min.push(x);
        }
    }
    
    // Pop 1: use equals.
    public void pop1() {
        // BUG 1: Very very trick. we should use EQUALS here instead of  "=="
        if (s.peek().equals(min.peek())) {
            min.pop();
        }
        s.pop();
    }
    
    // Pop 2: use int
    public void pop2() {
        // BUG 1: Very very trick. we should use EQUALS here instead of  "=="
        int n1 = s.peek();
        int n2 = min.peek();
        if (n1 == n2) {
            min.pop();
        }
        s.pop();
    }
    
    // Pop 3: use (int)
    public void pop() {
        // BUG 1: Very very trick. we should use EQUALS here instead of  "=="
        if ((int)s.peek() == (int)min.peek()) {
            min.pop();
        }
        s.pop();
    }

    public int top() {
        return s.peek();
    }

    public int getMin() {
        return min.peek();
    }
}

GITHUB:

https://github.com/yuzhangcmu/LeetCode_algorithm/blob/master/stack/MinStack.java