0155_min_stack.md

155. Min Stack

Easy

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.

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:

Methods pop, top and getMin operations will always be called on non-empty stacks.

Solution:

• Push a tuple of value and minimum at this point so far

/**
 * initialize your data structure here.
 */
var MinStack = function() {
    this.stack = [];
};

/** 
 * @param {number} x
 * @return {void}
 */
MinStack.prototype.push = function(x) {
    if (!this.stack.length) {
        this.stack.push([x, x]);
        return;
    }
    let min = Math.min(this.getMin(), x);
    this.stack.push([x, min]);
};

/**
 * @return {void}
 */
MinStack.prototype.pop = function() {
    return this.stack.pop()[0];
};

/**
 * @return {number}
 */
MinStack.prototype.top = function() {
    return this.stack[this.stack.length - 1][0];    
};

/**
 * @return {number}
 */
MinStack.prototype.getMin = function() {
    return this.stack[this.stack.length - 1][1];    
};

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