10大算法帮助你提高 JavaScript 技能

const find_missing = function(input) {  let n = input.length + 1;let sum = 0;  for (let i in input) {    sum += input[i];  }return Math.floor((n * (n + 1)) / 2) - sum;};

const reverse = function(num) {    let result = 0;    while (num !== 0) {      result = result * 10 + num % 10;      // Math.trunc() 方法会将数字的小数部分去掉，只保留整数部分      num = Math.trunc(num / 10);    }if (result > 2**31 || result < -(2**31)) return 0;    return result;};

const permute = function(nums) {    let results = [];let go = (current) => {      if (current.length === nums.length){        results.push(current);        return;      }      nums.forEach(n => {        if (!current.includes(n)){          go([...current, n]);        }      });    }    go([]);    return results;};

const checkPermutation = function(s1, s2) {  const len1 = s1.length, len2 = s2.length;  if (len1 > len2) return false;const count = Array(26).fill(0);  for (let i = 0; i < len1; i++) {      count[s1.charCodeAt(i)-97]++;      count[s2.charCodeAt(i)-97]--;  }  if (!count.some(e => e !== 0)) return true;for (let i = len1; i < len2; i++) {      count[s2.charCodeAt(i)-97]--;      count[s2.charCodeAt(i-len1)-97]++;      if (!count.some(e => e !== 0)) return true;  }  return false;};

const longestValidParentheses = function(S) {  let stack = [-1], ans = 0;  for (let i = 0; i < S.length; i++)    if (S[i] === '(') stack.push(i)    else if (stack.length === 1) stack[0] = i    else stack.pop(), ans = Math.max(ans, i - stack[stack.length-1])  return ans};

const fourSum = function(nums, target) {  let result = [];  let length = nums.length;  if (length < 4) return result;   nums = nums.sort((a, b) => a - b );for (let i = 0; i < length - 3; i++) {    if (nums[i] === nums[i - 1]) continue;    for (let j = i + 1; j < length - 2; j++) {      if (j > i + 1 && nums[j] === nums[j - 1]) continue;let k = j + 1;      let l = length - 1;while (k < l) {        const sum = nums[i] + nums[j] + nums[k] + nums[l];if (sum === target) {          result.push([nums[i], nums[j], nums[k], nums[l]])        }if (sum <= target) {          k += 1;          while (nums[k] === nums[k - 1]) {            k += 1;          }        }if (sum >= target) {          l -= 1;          while (nums[l] === nums[l + 1]) {            l -= 1;          }        }      }    }  }return result;};

const multiply = function(num1, num2) { if (num1 == 0 || num2 == 0) return ‘0’; const result = [];for (let a = num1.length — 1; a >= 0; a — ) { for (let b = num2.length — 1; b >= 0; b — ) { const p1 = a + b; const p2 = a + b + 1; const sum = (result[p2] ?? 0) + num1[a] * num2[b];result[p1] = (result[p1] ?? 0) + Math.floor(sum / 10); result[p2] = sum % 10; } } result[0] == 0 && result.shift(); return result.join(‘’);};

const shortestPalindrome = function(s) {  let index = 0;  for (let i = s.length - 1; i >= 0; i--) {    if (s[i] === s[index]) index++;  }  if (index === s.length) return s;  let remainingRev = s.substring(index, s.length);  console.log(remainingRev);  remainingRev = reverse(remainingRev);return remainingRev + shortestPalindrome(s.substring(0, index)) + s.substring(index);};function reverse(string) {  let myString = '';  for (let i = string.length - 1; i >= 0; i--) {    myString = myString + string[i];  }  return myString;};

const numberToWords = function(num) {  let result = toHundreds(num % 1000);  const bigNumbers = ["Thousand", "Million", "Billion"];  for (let i = 0; i < 3; ++i) {    num = Math.trunc(num / 1000);    result = num % 1000 !== 0 ? [toHundreds(num % 1000), bigNumbers[i], result].filter(Boolean).join(" ") : result;  }  return result.length === 0 ? "Zero" : result;}function toHundreds(num) {  const numbers = ["", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten",    "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"];  const tens = ["", "", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"];  const result = Array(3).fill("");  let a = Math.trunc(num / 100), b = num % 100, c = num % 10;  result[0] = a > 0 && `${numbers[a]} Hundred`;  result[1] = b < 20 ? numbers[b] : tens[Math.trunc(b / 10)]  result[2] = b >= 20 && `${numbers[c]}`;  return result.filter(Boolean).join(" ");}

const canConstruct = function(ransomNote, magazine) { if (ransomNote.length > magazine.length) return false; let magMap = new Map();for(let char of magazine) { magMap.set(char, (magMap.get(char) || 0 ) + 1); }for(let note of ransomNote) { let counter = magMap.get(note); if (!counter) return false;magMap.set(note, — counter); } return true;};