freeCodeCamp/curriculum/challenges/chinese/10-coding-interview-prep/rosetta-code/24-game.md

id, title, challengeType, forumTopicId, dashedName

id	title	challengeType	forumTopicId	dashedName
5951e88f64ebf159166a1176	24 点游戏	1	302218	24-game

--description--

The 24 Game tests a person's mental arithmetic.

游戏的目标是使四个数字的计算结果是 24。

--instructions--

实现一个函数，该函数将一个四位数的字符串作为其参数，每个数字从 1 到 9（含）并允许重复，并返回一个计算结果为数字 24 的算术表达式。 如果不存在这样的解，则返回“没有解决方案”。

规则：

• 只允许以下运算符/函数：乘法、除法、加法、减法。
• 除法应使用浮点或有理算术等来保留余数。
• 不允许从提供的数字中形成多位数字。 （所以当给出 1、2、2 和 1 时，12 + 12 的答案是错误的）。
• 不必依照给定的数字顺序。

示例输入	示例输出
solve24("4878");	(7-8/8)*4
solve24("1234");	3*1*4*2
solve24("6789");	(6*8)/(9-7)
solve24("1127");	(1+7)*(2+1)

--hints--

solve24 应该是一个函数。

assert(typeof solve24 === 'function');

solve24("4878") should return (7-8/8)*4, 4*(7-8/8), or a similar valid string

assert(isValidSolution_(solve24(testCases_[0])));

solve24("1234") should return 1*2*3*4 or a similar valid string

assert(isValidSolution_(solve24(testCases_[1])));

solve24("6789") should return (6*8)/(9-7), (8*6)/(9-7), or a similar valid string

assert(isValidSolution_(solve24(testCases_[2])));

solve24("1127") should return (1+7)*(1+2) or a similar valid string

assert(isValidSolution_(solve24(testCases_[3])));

--seed--

--after-user-code--

const testCases_ = [
  '4878',
  '1234',
  '6789',
  '1127'
];

const OPERATORS_ = {
  "+": (a, b) => a + b,
  "-": (a, b) => a - b,
  "*": (a, b) => a * b,
  "/": (a, b) => a / b,
}

const PRECEDENCE_ = {
  "+": 1,
  "-": 1,
  "*": 2,
  "/": 2,
}

function evaluate_(expression) {
  expression = expression.replace('/\s+/g', '');
  const stack = [];
  let postfix = "";

  // Convert from infix to postfix
  let head = 0;
  while (head < expression.length) {
    let c = expression[head];
    switch (c) {
      case "(":
        stack.push(c);
        break;
      case ")":
        let last = stack.pop();
        while (last !== "(") {
          postfix += last;
          last = stack.pop();
        }
        break;
      case "+":
      case "-":
      case "*":
      case "/":
        while (stack.length &&
               PRECEDENCE_[c] <= PRECEDENCE_[stack[stack.length-1]]) {
          postfix += stack.pop();
        }
        stack.push(c);
        break;
      case "0":
      case "1":
      case "2":
      case "3":
      case "4":
      case "5":
      case "6":
      case "7":
      case "8":
      case "9":
        postfix += c;
        break;
      default:
        return false;
    }
    head++;
  }

  // Clear out stack
  while (stack.length) {
    postfix += stack.pop();
  }

  // Evaluate postfix
  for (let c of postfix) {
    switch (c) {
      case "+":
      case "-":
      case "*":
      case "/":
        const b = +stack.pop();
        const a = +stack.pop();
        stack.push(OPERATORS_[c](a, b));
        break;
      default:
        stack.push(c);
    }
  }
  return stack.pop();
}

// Check solution validity
function isValidSolution_(userSolution) {
  return evaluate_(userSolution) === 24;
}

--seed-contents--

function solve24 (numStr) {

  return true;
}

--solutions--

function solve24(numStr) {
  const digitsArr = numStr.split('');
  const answers = [];

  const digitPermutations = [];
  const operatorPermutations = [];

  function generateDigitPermutations (digits, permutations = []) {
    if (digits.length === 0) {
      digitPermutations.push(permutations);
    }
    else {
      for (let i = 0; i < digits.length; i++) {
        const curr = digits.slice();
        const next = curr.splice(i, 1);
        generateDigitPermutations(curr.slice(), permutations.concat(next));
      }
    }
  }

  function generateOperatorPermutations (permutations = []) {
    const operators = ['+', '-', '*', '/'];
    if (permutations.length === 3) {
      operatorPermutations.push(permutations);
    }
    else {
      for (let i = 0; i < operators.length; i++) {
        const curr = permutations.slice();
        curr.push(operators[i]);
        generateOperatorPermutations(curr);
      }
    }
  }

  generateDigitPermutations(digitsArr);
  generateOperatorPermutations();

  interleave();

  return answers[0];

  function interleave () {
    for (let i = 0; i < digitPermutations.length; i++) {
      for (let j = 0; j < operatorPermutations.length; j++) {
        const d = digitPermutations[i];
        const o = operatorPermutations[j];
        const perm = [
          `${d[0]}${o[0]}${d[1]}${o[1]}${d[2]}${o[2]}${d[3]}`,
          `(${d[0]}${o[0]}${d[1]})${o[1]}${d[2]}${o[2]}${d[3]}`,
          `${d[0]}${o[0]}(${d[1]}${o[1]}${d[2]})${o[2]}${d[3]}`,
          `${d[0]}${o[0]}${d[1]}${o[1]}(${d[2]}${o[2]}${d[3]})`,
          `${d[0]}${o[0]}(${d[1]}${o[1]}${d[2]}${o[2]}${d[3]})`,
          `(${d[0]}${o[0]}${d[1]}${o[1]}${d[2]})${o[2]}${d[3]}`,
          `(${d[0]}${o[0]}${d[1]})${o[1]}(${d[2]}${o[2]}${d[3]})`
        ];

        perm.forEach(combination => {
          const res = eval(combination);

          if (res === 24) {
            return answers.push(combination);
          }
        });
      }
    }
  }
}