155. Min Stack

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:

MinStack minStack = new MinStack();
minStack.push(-2);
minStack.push(0);
minStack.push(-3);
minStack.getMin();   --> Returns -3.
minStack.pop();
minStack.top();      --> Returns 0.
minStack.getMin();   --> Returns -2.

解法：

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

/** 
 * @param {number} x
 * @return {void}
 */
MinStack.prototype.push = function(x) {
    var min=x;
    if(this.stack.length>0){
      min=Math.min(this.stack[this.stack.length-1].min,x);
    }
    this.stack.push({val:x,min:min})
};

/**
 * @return {void}
 */
MinStack.prototype.pop = function() {
    if(this.stack.length>0){
       return  this.stack.pop().val;
    }
};

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

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

/** 
 * 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()
 */