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Find missing integer problem

Problem : Given an array of size 99 which has integers from 1 to 100 and with no integer being repeated, can you find the integer which is not in the array ?

There are many differeny ways of solving this problem. The most efficient solution is to just find the difference between the sum of all integers from 1 to 100 and the sum of all integers in the array. This algorithm has a running complexity of O (n) and a space complexity of O (1) since it just needs 2 variables to store the sums. Here is the code for this -


  /**
     * This will find the missing integer in the given array of size 99.
     *
     * The array has integers between 1 and 100 with one integer
     * missing.
     *
     * @param array - Array of integers b/w 1 and 100 of size 99.
     * @return int - the missing integer number.
     */
    public static int findMissingInteger(int[] array)
    {
        int sum = 0;

        // This has the complexity of O (n)
        for (int i = 0; i <>
        {
            sum += array[i];
        }

        int sumOneToHundred = (100 * 101)  / 2;

        return sumOneToHundred - sum;
    }


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