偷个懒用eval 不想去判断符号

function evalRPN(tokens) {
  // write code here
  let flag = ['+', '-', '*', '/'];
  var res = [];
  let sum = 0;
  for (let i = 0; i < tokens.length; i++) {
    let index = flag.indexOf(tokens[i]);
    if (index !== -1) {
      let r1 = res.pop();
      let r2 = res.pop();
      let opr = flag[index];
      sum = eval(`${r2}${opr}(${r1})`);
      res.push(parseInt(sum+''))
    } else {
      res.push(tokens[i]);
    }
  }
   return res.pop();

}