MissingInteger.java

package MissingInteger;

import java.util.*;

class Solution {
    public int solution(int[] A) {
        // write your code in Java SE 8
        
        //special case
        if(A.length ==0){
            return 1;
        }
        
        // Using "set" to check if an element has appeared
        // note: need to "import java.util.*" (important)
        Set<Integer> set = new HashSet<Integer>();
        
        // add elements into the set
        for(int i=0; i< A.length; i++){
            set.add(A[i]);
        }
        
        // note: the missing number is not possible bigger than (A.length)
        //       because there are only (A.length) numbers
        for(int i=1; i<= A.length; i++){
            if(set.contains(i) != true) // the 1st missing element
                return i;
        }
        
        // means: there are no missing numbers from 1 to A.length
        // Therefore, the missing number is "A.length+1" (important)
        return A.length+1;
    }
}