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 );