PermMissingElem

An array A consisting of N different integers is given. The array contains integers in the range [1..(N + 1)], which means that exactly one element is missing.

Your goal is to find that missing element.

Write a function:

function solution(A);

that, given an array A, returns the value of the missing element.

For example, given array A such that:

A[0] = 2 A[1] = 3 A[2] = 1 A[3] = 5

the function should return 4, as it is the missing element.

Write an efficient algorithm for the following assumptions:

• N is an integer within the range [0..100,000];
• the elements of A are all distinct;
• each element of array A is an integer within the range [1..(N + 1)].

// you can write to stdout for debugging purposes, e.g.
// console.log('this is a debug message');

function solution(A) {
    // write your code in JavaScript (Node.js 8.9.4)
    if(A.length === 0){
        return 1;
        }else {
    let sumlength = ((1+ A.length+1)*(A.length+1))/2;
    let res = A.reduce((e1, e2) => e1 + e2);
    return sumlength - res ;
        }
}

reduce로 callback 함수 하나 만들기

연속 수 누적합: N(N+1)/2

 

N+1꺼 계산하고 더한 값 빼면 없는애가 튀어 나옴.