155. Min Stack

Description of Problem

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(int 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. You must implement a solution with O(1) time complexity for each function.

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:

• -2^31 <= val <= 2^31 - 1
• Methods pop, top and getMin operations will always be called on non-empty stacks.
• At most 3 * 10^4 calls will be made to push, pop, top, and getMin.

Solution

Tags: Stack

Explanation

Store the current minimum element on each element in the stack. And therefore, the top of the stack must contain the current minimum number.

Code

struct MinStack {
    // (value, min value it knows) 
    stack : Vec<(i32,i32)>
}


/** 
 * `&self` means the method takes an immutable reference.
 * If you need a mutable reference, change it to `&mut self` instead.
 */
impl MinStack {

    fn new() -> Self {
        MinStack {
            stack: Vec::new()
        }
    }
    
    fn push(&mut self, val: i32) {
        if self.stack. len() > 0 {
            self.stack.push((
                val, val.min( self.get_min() )
            ));
        } else {
            self.stack.push( (val, val));
        }
    }
    
    fn pop(&mut self) {
        assert!(self.stack.len() > 0);
        self.stack.pop();
    }
    
    fn top(&self) -> i32 {
        assert!(self.stack.len() > 0);
        let n = self.stack. len();
        return self.stack[n - 1].0;
    }
    
    fn get_min(&self) -> i32 {
        assert! (self.stack.len() > 0);
        let n = self.stack. len();
        return self.stack[n - 1].1;
    }
}

/**
 * Your MinStack object will be instantiated and called as such:
 * let obj = MinStack::new();
 * obj.push(val);
 * obj.pop();
 * let ret_3: i32 = obj.top();
 * let ret_4: i32 = obj.get_min();
 */

Complexity

Time Complexity

• \(T(n) = O(1)\)

Auxiliary Space

• \(S(n) = O(1)\)