text: Count ways to reach Nth Stairs by taking 1 and 2 steps with exactly one 3 step | C # implementation to find the number the number of ways to reach Nth stair by taking 1 , 2 step at a time and 3 Steps at a time exactly once . ; Function to find the number the number of ways to reach Nth stair ; Array including number of ways that includes 3 ; Array including number of ways that doesn 't includes 3 ; Initially to reach 3 stairs by taking 3 steps can be reached by 1 way ; Loop to find the number the number of ways to reach Nth stair ; Driver Code
code: using System ; class GFG { static int number_of_ways ( int n ) { int [ ] includes_3 = new int [ n + 1 ] ; int [ ] not_includes_3 = new int [ n + 1 ] ; includes_3 [ 3 ] = 1 ; not_includes_3 [ 1 ] = 1 ; not_includes_3 [ 2 ] = 2 ; not_includes_3 [ 3 ] = 3 ; for ( int i = 4 ; i <= n ; i ++ ) { includes_3 [ i ] = includes_3 [ i - 1 ] + includes_3 [ i - 2 ] + not_includes_3 [ i - 3 ] ; not_includes_3 [ i ] = not_includes_3 [ i - 1 ] + not_includes_3 [ i - 2 ] ; } return includes_3 [ n ] ; } public static void Main ( String [ ] args ) { int n = 7 ; Console . Write ( number_of_ways ( n ) ) ; } }