Solution.ts

class StackOfPlates {
    private cap: number;
    private stacks: number[][];

    constructor(cap: number) {
        this.cap = cap;
        this.stacks = [];
    }

    push(val: number): void {
        if (this.cap === 0) {
            return;
        }
        const n = this.stacks.length;
        const stack = this.stacks[n - 1];
        if (stack == null || stack.length === this.cap) {
            this.stacks.push([val]);
        } else {
            stack.push(val);
        }
    }

    pop(): number {
        const n = this.stacks.length;
        if (n === 0) {
            return -1;
        }
        const stack = this.stacks[n - 1];
        const res = stack.pop();
        if (stack.length === 0) {
            this.stacks.pop();
        }
        return res;
    }

    popAt(index: number): number {
        if (index >= this.stacks.length) {
            return -1;
        }
        const stack = this.stacks[index];
        const res = stack.pop();
        if (stack.length === 0) {
            this.stacks.splice(index, 1);
        }
        return res;
    }
}

/**
 * Your StackOfPlates object will be instantiated and called as such:
 * var obj = new StackOfPlates(cap)
 * obj.push(val)
 * var param_2 = obj.pop()
 * var param_3 = obj.popAt(index)
 */