Flattening a 2D array to a 1D array based on indexes

I am certain there is a better way to flatten the array with an optimized approach for the solution.

Example:


[[1,2,24],[12,1241,122],[3,2],[7]] => 1 12 3 7 2 1241 2 24 122

public class flattenArray {

public static void main(String args[]) {

    int inputLists[][] = {{1,2,24},{12,1241,122},{3,2},{7}};

    flattenList(inputLists);

}

public static void flattenList(int input[][]){
    List result = new ArrayList();
    Map> elements = new HashMap>();
    for(int i=0;i temp = new ArrayList();

            if(elements.get(j)!=null) {
                temp=elements.get(j);
                temp.add(input[i][j]);
                elements.put(j, temp);
            }

            else {
                temp.add(input[i][j]);
                elements.put(j,temp);
            }
        }
    }

    for(int i=0;i<input.length;i++){
        if(elements.get(i)!=null)
        result.addAll(elements.get(i));
    }

    for(int i=0;i<result.size();i++){
        System.out.println(result.get(i)+" ");
    }
}
}

Don't use raw types:

List result = new ArrayList();

Should be:

List result = new ArrayList<>();

If you are using Java8, you could simplify the code of the map creation using the function computeIfAbsent:

for (int i = 0; i  new ArrayList<>()).add(input[i][j]);
    }
}

In the next loop, you are using the break condition incorrectly.

for(int i=0;i<input.length;i++){ // input.length is not what you need 
    if(elements.get(i)!=null)
        result.addAll(elements.get(i));
}

input.length is the number of arrays in your 2d array. If the length of any array is greater than the number of arrays, the code is skipping values.

This is a failed test:

Input: {{1,2,24},{12,1241,122}} 
Output: 1 12 2 1241
Expected: 1 12 2 1241 24 122

You don't need to use input in the loop, you have a map with all the elements for each index, so you can make the conversion using only the map:

// Use a TreeMap instead of a HashMap to have the elements ordered by index
Map> elements = new TreeMap>();
...
for (List e : elements.values()) {
    result.addAll(e);
}

StackExchange Code Review Q#139480, answer score: 5