Here is my solution in java, time complexity O(n^2)`

public static void staircase(int n) {
    // Write your code here
    for(int i=0;i<n;i++)
    {
        for(int j=0;j<n;j++)
        {   
            if(j<n-(i+1))
            System.out.print(" ");
            else
            System.out.print("#");
        }
        System.out.println();
    }
    }