155.Min Stack.md

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155.Min Stack

Easy

https://leetcode.com/problems/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 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.

解题：

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

/** 
 * @param {number} x
 * @return {void}
 */
MinStack.prototype.push = function(x) {
    this.stack.push(x)
    // set new min
    if(this.minVal == null) {
        this.minVal = x
    } else {
        this.minVal = Math.min(this.minVal, x)
    }
};


/**
 * @return {void}
 */
MinStack.prototype.pop = function() {
    this.stack.pop()
    // find new min
    this.minVal = Number.MAX_VALUE
    for(let i = 0; i < this.stack.length; i++) {
        this.minVal = Math.min(this.stack[i], this.minVal)
    }
};

/**
 * @return {number}
 */
MinStack.prototype.top = function() {
    const len = this.stack.length
    let res = null
    if(len > 0) {
        res = this.stack[len-1]
    }
    return res
    
};

/**
 * @return {number}
 */
MinStack.prototype.getMin = function() {
    return this.minVal
};

/** 
 * 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()
 */