freecodecamp算法题解系列（三）--中级算法（11-21）

Intermediate Algorithm

承接上篇，本篇继续总结freecodecamp中级算法的第11-21题。

11. Convert HTML Entities

Convert the characters &, <, >, “ (double quote), and ‘ (apostrophe), in a string to their corresponding HTML entities.
将HTML的特殊字符转义

function convert(str) {
    // :)
    return str.replace(/&/g,"&amp;").replace(/</g,"&lt;").replace(/>/g,"&gt;").replace(/\"/g,"&quot;").replace(/\'/g,"&apos;");
}

convert("Hamburgers < Pizza < Tacos");

12. Spinal Tap Case

Convert a string to spinal case. Spinal case is all-lowercase-words-joined-by-dashes.

字符串格式变换，将给定的字符串用”-“连接起来。

function spinalCase(str) {
    // "It's such a fine line between stupid, and clever."
    // --David St. Hubbins
    //return str.toLowerCase().replace(/[^a-zA-Z]/g," ").replace(/\s/g,'-');
    if(!/[^a-zA-Z]/.test(str) && str.indexOf(" ") === -1){
        var res = "";
        for(var i=0,len=str.length;i<len;i++){
            if(/[A-Z]/.test(str[i])){
                var tmp = " " + str[i];
                res += tmp;
            }else {
                res += str[i];
            }
        }
        str = res;
    }
      
    return str.toLowerCase().replace(/[^a-zA-Z]/g," ").replace(/\s/g,'-');
}

spinalCase('This Is Spinal Tap');
spinalCase("thisIsSpinalTap") ;
spinalCase("The_Andy_Griffith_Show");

13. Sum All Odd Fibonacci Numbers

Return the sum of all odd Fibonacci numbers up to and including the passed number if it is a Fibonacci number.

求和给定参数内的所有奇Fibonacci数的和。Fibonacci数是后一个数字等于前两个数字和的形式的数列。

/* 递归方式
 * function fibNums(num){
       //返回所有小于num的fib数
       if(num <= 2){
           return 1;
       }else {
           return fibNums(num-2) + fibNums(num-1);
       }
 }*/
 function fibNums(num){
       //非递归方式
       var tmp = [1,1];
       if(num <=2){
           return 1;
       }
       for(var i=3;i<=num;i++){
           var t = tmp[0] + tmp[1];
           tmp[1] = tmp[0];
           tmp[0] = t;
       }
       return tmp[0];
 }

 //求小于num的fibnacci数
 function sumFibs(num){
     var sum = 0;
     for(var i=1;fibN = fibNums(i), fibN <= num;i++){
         if(fibN %2){
             sum += fibN;
         }
     }
     return sum;
 }



 sumFibs(1000);

14. Sum All Primes

Sum all the prime numbers up to and including the provided number.

求和给定参数内的所有素数。

function isPrime(num){
      var sum = 0;
      for(var i=2;i<=Math.sqrt(num);i++){
          sum += i;
          if(num % i === 0){
                return false;
          }
      }
      return true;
}

//后续算法改进
function sumPrimes(num) {
      var sum = 0;
      for(var i=2;i<=num;i++){
          if(isPrime(i)){
              sum += i;
          }
      }
      return sum;
}


sumPrimes(10);

15. Smallest Common Multiple

Find the smallest common multiple of the provided parameters that can be evenly divided by both, as well as by all sequential numbers in the range between these parameters.

给定一个参数数组，返回数组序列内所有数字的最小公倍数。如给定[1,5]，则需要返回”1,2,3,4,5”的最小公倍数。

function smallestCommon(m,n){
      var max = m > n ? m : n;
      for(var i=max;;i+=max){
          if((i%m === 0) && (i%n === 0)){
                return i;
          }
      }
}

//分治给出数组的最小公倍数
function divide2solve(arr){
      if(arr.length === 1 ){
          return smallestCommon(1,arr[0]);
      }
      var mid = arr.length / 2;
      var left = arr.slice(0,mid);
      var right = arr.slice(mid);
      var leftCommon = divide2solve(left);
      var rightCommon = divide2solve(right);
      return smallestCommon(leftCommon,rightCommon);
}

function smallestCommons(arr) {
      var res = [];
      var start = arr[0] < arr[1] ? arr[0] : arr[1],
          end = arr[0] < arr[1] ? arr[1] : arr[0];
      for(;start <= end;start++){
          res.push(start);
      }
      return divide2solve(res);
}

smallestCommons([1,5]);

16. Finders Keepers

Create a function that looks through an array (first argument) and returns the first element in the array that passes a truth test (second argument).

给定两个参数，第一个参数是一个数组，第二个参数是一个测试函数，算法需要返回满足该测试函数的第一个数组元素。

function find(arr, func) {
      var res = arr.filter(func);
      if(res.length){
          return res[0];
      }
      return undefined;
}


find([1, 2, 3, 4], function(num){ return num % 2 === 0; });
find([1, 3, 5, 8, 9, 10], function(num) { return num % 2 === 0; });

17. Drop it

Drop the elements of an array (first argument), starting from the front, until the predicate (second argument) returns true.

给定两个参数，第一个参数是一个数组，第二个参数是一个测试函数，算法需要以数组形式弹出满足测试函数的数组元素。

function drop(arr, func) {
      // Drop them elements.
      var tmp;
      var res = [];
      for(var i=0,len=arr.length;i<len;i++){
          tmp = arr.shift();
          if(func(tmp)){
            //需要置回弹出的元素
            arr.unshift(tmp);
            break;
          }
      }
      
      return arr;
}

drop([1, 2, 3], function(n) {return n < 3; });
drop([1, 2, 3, 5], function(n) {return n > 5;});

18. Steamroller

Flatten a nested array. You must account for varying levels of nesting.

“Steamroller”,压路机，很形象，将一个嵌套多层的数组扁平化，亦即接受多层嵌套的数组，输出无嵌套数组，且输出数组的元素为输入嵌套的数组元素。

function steamroller(arr, flatArr) {
  if (!flatArr)    flatArr = [];

  for (var i in arr) {
    if(!Array.isArray(arr[i])){
      flatArr.push(arr[i]);
    }else {
      steamroller(arr[i],flatArr);
    }
   
  }

  return flatArr;
}

steamroller([1, [2], [3, [[4]]]]);

19. Binary Agents

Return an English translated sentence of the passed binary string.

翻译给定的二进制串，给定一串01串，输出表示的句子。

function trans(str){
  var sum =0;
  for(var i=0,len=str.length;i<len;i++){
    (function (index){
        sum += str[index] * Math.ceil(str[index] * Math.pow(2,len-1-index));
    })(i);
  }
  return sum;
}


function binaryAgent(str) {
  var res = "";
  var arr = str.split(" ");
  for(var i in arr){
    var ret = trans(arr[i]);
    res += String.fromCharCode(ret);
  }
  return res;
}

binaryAgent("01000001 01110010 01100101 01101110 00100111 01110100 00100000 01100010 01101111 01101110 01100110 01101001 01110010 01100101 01110011 00100000 01100110 01110101 01101110 00100001 00111111");
//binaryAgent("01000001");

20. Everything Be True

Check if the predicate (second argument) is truthy on all elements of a collection (first argument).

给定两个参数，第一个参数是一个对象数组，第二个参数是一个字符串，算法需要判断参数1的对象数组是否都有参数2对应的key且对应的value值为true。

function every(collection,pre){
  for(var i in collection){
    if(!collection[i][pre]){
      return false;
    }
  }
  return true;
}

//every([{"user": "Tinky-Winky", "sex": "male"}, {"user": "Dipsy", "sex": "male"}, {"user": "Laa-Laa", "sex": "female"}, {"user": "Po", "sex": "female"}], "sex");
every([{"user": "Tinky-Winky", "sex": "male", "age": 0}, {"user": "Dipsy", "sex": "male", "age": 3}, {"user": "Laa-Laa", "sex": "female", "age": 5}, {"user": "Po", "sex": "female", "age": 4}], "age");

21. Arguments Optional

Create a function that sums two arguments together. If only one argument is provided, then return a function that expects one argument and returns the sum.

js可选参数问题，题干要求求和给定的两个参数，如果只给定了一个参数，则返回一个可以再接收一个参数进行求和的函数。

function isNum(val){
  return typeof val === "number";
}

function add(x) {
  if(arguments.length == 1){
    if(!isNum(x)){
      return undefined;
    }
    return function(y){
      if(!isNum(y)){
        return undefined;
      }
      return x + y;
    };
  }
  if(!isNum(arguments[1])){
    return undefined;
  }
  return arguments[0] + arguments[1];
}

add(2)([3]);