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