Reducing the number of repeating same strings or integers to 1

Initial:

 list = { null, 1, 2, null, null, null, 4, 5, 6 }; 

Expectatitons:

 list = { null, 1, 2, null, 4, 5, 6 };

>Solution :

If list is an obsolete ArrayList or List<oblect> you can try a simple for loop:

var list = new ArrayList { null, 1, 2, null, null, null, 4, 5, 6 };

...

int index = 0;

// Remove consequent null's
for (int i = 1; i < list.Count; ++i)
    if (list[i] != null || list[index] != null)
        list[index++] = list[i];

list.RemoveRange(index, list.Count - index);

To get rid of consequent duplicates

int index = 0;

for (int i = 1; i < list.Count; ++i)
    if (list[i] != list[index])
        list[index++] = list[i];

list.RemoveRange(index, list.Count - index);