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/*

** The problem could be solved by dynamic problem, if we see the stairs as an array,

** we could calculate from back to front. Each steps required in a stair to reach top

** is the sum of next 2 stairs.

**

** Finally, we would recognize it is just Fibonacci Sequence (just step 1 or 2),

* the result is merely the

** nth value in the sequence.

**

*/

public class ClimbStairs {

public static void main(String[] args) {

int N = 5;

System.out.println(climbStairs(N));

}

public static int climbStairs(int n) {

if (0 < n && n <= 1) return n;

else if (n <= 0) return 1;

else return climbStairs(n - 1)

+ climbStairs(n - 2)

+ climbStairs(n - 3); // add the last step 1 or 2 or 3.

}

}