题解 | #【模板】栈#

let stack = [];
let top = -1;
while ((line = readline())) {
  var arr = line.split(" ");
  // 将循环的次数跳过  
  if (!isNaN(parseInt(arr[0]))) {
    continue;
  }
  if (arr.length === 2) {
    stack[++top] = arr[1];
  } else if (arr[0] === "pop") {
    if (top === -1) {
      console.log("error");
    } else {
      console.log(stack[top--]);
    }
  } else {
    if (top === -1) {
      console.log("error");
    } else {
      console.log(stack[top]);
    }
  }
}