How to find the missing numbers from an array of integers. Q: You’re given an array of N integers. N may be very large. You know that every integer 1-N appears once in the array, except there is one or more integer(s) missing.

missing_int_from_array.js

/* ============================
 *    EXAMPLE 1:
 * ============================ */
var numbers = [0,1,3,4,5,7,8]; // Missing 2,6
var lastNumber = numbers[numbers.length - 1];
var expectedSum = (lastNumber * (lastNumber + 1)) / 2;
var actualSum = 0;

// Show the difference
for (var i = 0; i < numbers.length; i++) {
    actualSum += numbers[i];
}

console.log(expectedSum - actualSum);


/* ============================
 *    EXAMPLE 2:
 * ============================ */
var numbers = [0,1,3,4,5,7,8]; // Missing 2,6
var missing = [];

// Find the missing array items
for ( var i = 0; i < numbers.length; i++ ) {

    if ( (numbers[i+1] - numbers[i]) > 1 ) {
        missing.push( numbers[i+1] - numbers[1] );   
    }
}

console.log( missing );


/* ============================
 *    EXAMPLE 3:
 * ============================ */
var intArray = [9,1,5,8,7,4,3,0,10,13,15,19,12,16,18];  // Missing 2,6,11,14,17
var arrayLength = Math.max.apply(Math, intArray);
var missing = []

for ( var i = 0; i < arrayLength; i++ ) {
    if ( intArray.indexOf(i) < 0 ) {
        missing.push( i );
    }
}

console.log( missing );

I don't know about the last one...as I don't know Javascript (Math.max.apply(...)). But the middle one seems to have a bug in that it cannot detect missing array indexes of multiple missing numbers next to one another. I have only done paper and pen, but...

var numbers = [1, 2, 3, 6, 7, 8];  // Missing 4, 5 ... index 3,  4

reports only index 3...but should also report index 4 (if I am right about what you are suppose to do)

In Java:

for (int i = 0;  i < numbers.length; i++) {

    int diff = numbers[i + 1] - numbers[i];
    int count = 0;

    while ((diff - count) > 1) {

        missing.push(diff + count);
        count++;

    }
}

I have not tested this...but this seems to work

Only tested the first example which seems to work if the array contains only single digits. For instance in this case
var numbers = [10, 11, 12, 14];
(14 * 15) / 2 = 105
10 + 11 + 12 + 14 = 47
105 - 47 = 58

/Example Three just sort the array/

var missingNumber = function(nums) {
var out = [];
nums.sort(function(a,b){return a-b});
for(var i=0;i<nums.length;i++) {
if(nums[i+1] - nums[i] > 1) {
out.push(nums[i+1] - nums[1]);
}
}
return out;
};

the best one-line solution I came up with so far is
missingnums = (Math.max.apply(Math, x) - Math.min.apply(Math, x) + 1) - x.length