# 10个算法提升你的JavaScript技能
**1)在数组中查找缺失的数字**
```text
Input: [1, 2, 3, 4, 6, 7, 8, 9, 10]
Output: 5
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;
};
```
提示：算术级数和公式：
**2)反转整数**
```text
Input: num = 123
Output: 321
Input: num = -123
Output: -321
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;
};
```
**3) 数组排列**
```text
Input: [1,2,3]
Output: [[1,2,3],[1,3,2],[2,1,3],[2,3,1],[3,1,2],[3,2,1]]
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;
};
```
**4) 字符串中的排列**
```text
Input: s1 = "ab", s2 = "eidbao"
Output: true
Input: s1 = "aa", s2 = "eidbao"
Output: false
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;
};
```
**5) 最长有效括号**
```text
Input: "(()"
Output: 2
Input: ")()())"
Output: 4
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
};
```
**6) 4Sum**
```text
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;
};
```
**7)字符串相乘**
```text
Input: num1 = "2", num2 = "3"
Output: "6"
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('');
};
```
**8) 最短回文**
```text
Input: s = "aacecaaa"
Output: "aaacecaaa"
Input: s = "abcd"
Output: "dcbabcd"
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;
};
```
**9)整数到英文单词**
```text
Input: num = 123
Output: "One Hundred Twenty Three"
Input: num = 1234567
Output: "One Million Two Hundred Thirty Four Thousand Five Hundred Sixty Seven"
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(" ");
}
```
**10) 赎金票据**
```text
Input: ransomNote = "aa", magazine = "ab"
Output: false
Input: ransomNote = "aa", magazine = "aab"
Output: true
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;
};
```
经过一番研究，我得出了这 10 个算法，每个算法都有特殊的技巧，不仅可以提高
我们的 JavaScript 技能，还可以帮助我们提高批判性思维能力。
**- End -**
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