给定数字、重复、POP、+、-的Javascript字符串，返回堆栈的最后一个数字

function calculate(input) {
  const stack = [];

  for (let i = 0; i < input.length; i++) {
    const char = input[i];

    if (!isNaN(Number(char))) {  // 如果是数字
      stack.push(Number(char));
    } else if (char === 'REP') {  // 如果是重复字符
      const lastNum = stack[stack.length - 1];
      stack.push(lastNum);
    } else if (char === '+') {  // 如果是'+'
      const num2 = stack.pop();
      const num1 = stack.pop();
      stack.push(num1 + num2);
    } else if (char === '-') {  // 如果是'-'
      const num2 = stack.pop();
      const num1 = stack.pop();
      stack.push(num1 - num2);
    }
  }

  return stack[stack.length - 1];  // 返回堆栈的最后一个数字
}

const input = "4 5 + 3 -";
const result = calculate(input);
console.log(result);  // 输出: 6