一些递归的练习题

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一些递归的练习题

每道题前面大概会留个十来行空行所以答案会被折叠这样……?

  1. 数字和

    /**
    
    
    
    
    
    
    
    
    
    
    
    */
    // find the sum of digits of a positive integer number using recursion
    // 35 => 3 + 5
    const sumOfDigits = (num: number) => 
      if (num < 0) throw new Error('input should be a positive integer');
    
      let sum = 0;
    
      if (num === 0) return 0;
    
      sum += (num % 10) + sumOfDigits(Math.floor(num / 10));
    
      return sum;
    ;
    
    console.log(sumOfDigits(35));
    console.log(sumOfDigits(45));
    
  2. 数字的乘方

    /**
    
    
    
    
    
    
    
    
    
    
    
    */
    // calculate the power of a number
    const powOfNum = (num: number, pow: number) => 
      if (!Number.isInteger(pow)) throw new Error('power can only be integer');
    
      // base case, all number's pow of 0 is 1
      if (pow === 0) return 1;
    
      if (pow < 0) return (1 / num) * powOfNum(num, pow + 1);
    
      return num * powOfNum(num, pow - 1);
    ;
    
    console.log(powOfNum(2, 2));
    console.log(powOfNum(2, -2));
    console.log(powOfNum(0.5, 2));
    
  3. 最大公约数

    用的是欧几里得算法

    /**
     
    
    
    
    
    
    
    
    
    
    
    */
    // euclidean algorithm
    const gcd = (a: number, b: number) => 
      if (!(Number.isInteger(a) && Number.isInteger(b)))
        throw new Error('both numbers must be integer');
      if (b === 0) return a;
    
      if (a < 0) a = -a;
    
      if (b < 0) b = -b;
    
      return gcd(b, a % b);
    ;
    
    console.log(gcd(48, 18));
    console.log(gcd(48, -18));
    
  4. 十进制转二进制

    没有做 2’s complement

    /**
    
    
    
    
    
    
    
    
    
    
    
    */
    // euclidean algorithm
    const decimalToBinary = (num: number) => 
      // did not implement 2's complement
      if (!Number.isInteger(num)) throw new Error('input should be an integer');
    
      if (num == 0) return 0;
    
      return (num % 2) + decimalToBinary(Math.trunc(num / 2)) * 10;
    ;
    
    console.log(decimalToBinary(10));
    console.log(decimalToBinary(-10));
    
  5. 阶乘

    /**
    
    
    
    
    
    
    
    
    
    
    
    */
    const factorial = (num: number) => 
      if (!Number.isInteger(num)) throw new Error('input should be an integer');
    
      if (num == 1) return 1;
    
      return num * factorial(num - 1);
    ;
    
    console.log(factorial(4));
    
  6. 数组的乘积和

    /**
    
    
    
    
    
    
    
    
    
    
    
    */
    const productOfArray = (arr: number[]) => 
      if (!arr) throw new Error('input should be an array');
    
      if (arr.length == 1) return arr[0];
    
      return arr[0] * productOfArray(arr.slice(1));
    ;
    
    console.log(productOfArray([1, 2, 3]));
    console.log(productOfArray([1, 2, 3, 4]));
    console.log(productOfArray([1, 2, 3, 10]));
    
  7. 获得区间和

    0-n 的和

    /**
    
    
    
    
    
    
    
    
    
    
    
    */
    // accepts a number and adds up all the numbers from 0 to the number passed to the function
    const getRange = (num: number) => 
      if (!Number.isInteger(num) || num < 0)
        throw new Error('input should be a positive integer');
    
      if (num === 0) return num;
    
      return num + getRange(num - 1);
    ;
    
    console.log(getRange(6));
    console.log(getRange(10));
    
  8. 斐波那契数字

    /**
    
    
    
    
    
    
    
    
    
    
    
    */
    const fibNum = (num: number) => 
      if (!Number.isInteger(num) || num < 0)
        throw new Error('input should be a positive integer');
    
      if (num < 2) return num;
    
      return fibNum(num - 1) + fibNum(num - 2);
    ;
    
    console.log(fibNum(4));
    console.log(fibNum(10));
    console.log(fibNum(28));
    
  9. 翻转字符串

    /**
    
    
    
    
    
    
    
    
    
    
    
    */
    const reverseStr = (str: string) => 
      if (!str.length) return '';
    
      return reverseStr(str.slice(1)) + str[0];
    ;
    
    console.log(reverseStr('javascript'));
    console.log(reverseStr('typescript'));
    
  10. 是否为回文字符串

    /**
    
    
    
    
    
    
    
    
    
    
    
    */
    const isPalindrome = (str) => 
      if (typeof str !== 'string') throw new Error('input should be a string');
    
      if (!str.length || str.length === 1) return true;
    
      return (
        str[0] === str[str.length - 1] &&
        isPalindrome(str.substring(1, str.length - 1))
      );
    ;
    
    console.log(isPalindrome('abcba'));
    console.log(isPalindrome('tacocat'));
    console.log(isPalindrome('abbcad'));
    
  11. some 递归调用回调函数

    /**
    
    
    
    
    
    
    
    
    
    
    
    */
    // array.some(cb)
    const someRecursiveCallback = (
      arr: number[],
      cb: (num: number) => boolean
    ) => 
      if (!arr) throw new Error('input should be an array');
    
      if (arr.length === 1) return cb(arr[0]);
    
      if (!cb(arr[0])) return someRecursiveCallback(arr.slice(1), cb);
    
      return true;
    ;
    
    console.log(someRecursiveCallback([1, 2, 3, 4, 5], isOdd));
    console.log(someRecursiveCallback([2, 4, 6], isOdd));
    
  12. flat 数组

    /**
    
    
    
    
    
    
    
    
    
    
    
    */
    // arr.flat()
    const flatten = (arr: any) => 
      arr.flat;
    
      if (!arr) throw new Error('input should be an array');
      const res: any[] = [];
    
      for (const val of arr) 
        if (val instanceof Array) res.push(...flatten(val));
        else res.push(val);
      
    
      return res;
    ;
    
    console.log(flatten([1, [2, [3, [4, [5], 6]]]]));
    
  13. 大写数组中的首字母

    /**
    
    
    
    
    
    
    
    
    
    
    
    */
    const capitalizeFirst = (arr) => 
      if (!arr) throw new Error('input should be an array');
    
      if (!arr.length) return [];
    
      return [arr[0][0].toUpperCase() + arr[0].substring(1)].concat(
        ...capitalizeFirst(arr.slice(1))
      );
    ;
    
    console.log(capitalizeFirst(['car', 'wash', 'wasabi']));
    
  14. 计算对象中出现的和

    /**
    
    
    
    
    
    
    
    
    
    
    
    */
    const obj1 = 
      outer: 2,
      obj: 
        inner: 2,
        otherObj: 
          superInner: 2,
          notANumber: true,
          alsoNotANumber: 'yup',
        ,
      ,
    ;
    
    const obj2 = 
      a: 2,
      b:  b: 2, bb:  b: 3, bb:  b: 2   ,
      c:  c:  c: 2 , cc: 'ball', ccc: 5 ,
      d: 1,
      e:  e:  e: 2 , ee: 'car' ,
    ;
    
    const nestedEvenSum = (obj: any, sum = 0) => 
      for (const [, val] of Object.entries(obj)) 
        if (typeof val === 'number' && !isOdd(val)) sum += val;
        else if (val instanceof Object) sum += nestedEvenSum(val);
      
    
      return sum;
    ;
    
    console.log(nestedEvenSum(obj1));
    

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