从1..N items数组中找到缺少的项目

Question

I was asked to find the missing number from 1..N array.

For instance, for array: let numArr = [2,4,6,8,3,5,1,9,10]; the missing number is 7

let numArr=[2,4,6,8,3,5,1,9,10];
numArr.sort(function(a,b){  //sort numArr
  return a-b;
});

let newNumArr=[];
for(let i=1;i<=10;i++){
  newNumArr.push(i);
}

for(let i=0;i<newNumArr.length;i++){  //compare with new arr
  if(newNumArr[i] !== numArr[i]){
    console.log('The missing num is:'+newNumArr[i]);  //The missing num is:7
    break;
  }
}

Answer 1

You can use MAP and FILTER to find out the missing number in seperate array

const numArr = [2, 4, 6, 8, 3, 5, 1, 9, 10];
const missingNumberArray = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10].map(number => {
    if (!numArr.includes(number)) {
        return number;
    }
}).filter(y => y !== undefined);

Answer 2

You can use the simple logic of sum of consecutive n numbers is n*(n+1)/2 . Subtracting the sum of array numbers from above will give the missing number

 let numArr=[2,4,6,8,3,5,1,9,10]; var sum = numArr.reduce((a,c) => a+c, 0); // As the array contains n-1 numbers, here n will be numArr.length + 1 console.log(((numArr.length + 1) * (numArr.length + 2))/2 - sum); 

Answer 3

It would be easier to use .find :

 function findMissing(input) { input.sort((a, b) => a - b); const first = input[0]; return input.find((num, i) => first + i !== num) - 1; } console.log(findMissing([2, 4, 6, 8, 3, 5, 1, 9, 10])); console.log(findMissing([3, 4, 5, 6, 8, 9, 2])); 

(note that this also works for finding missing values from arrays that don't start at 1)

Answer 4

You can use XOR features.

• XOR all the array elements, let the result of XOR be arr_xor .
• XOR all numbers from 1 to n, let XOR be interval_xor .
• XOR of arr_xor and interval_xor gives the missing number.

 let numArr=[2,4,6,8,3,5,1,9,10]; function getMissingNo(arr){ arr = arr.sort() n = arr.length arr_xor = arr[0] interval_xor = 1 for(i = 0; i < n; i++) arr_xor ^= arr[i] for( i = 0; i<n + 2; i++) interval_xor ^= i return arr_xor ^ interval_xor } console.log(getMissingNo(numArr));