下楼梯

当然， 这里需要优化，空间换时间。

public class Steps {public int invokeTimes;private int[] a;public Steps(int n){assert(n>0);a = new int[n+1];}public int f(int n){invokeTimes++;if(a[n] > 0){return a[n];}if(n == 1){return 1;}else if(n == 2){return 2;}int count = f(n-1) + f(n-2);a[n] = count;return count;}/** * @param args */public static void main(String[] args) {Steps s = new Steps(30);int count = s.f(30);System.out.println(count);System.out.println(s.invokeTimes);}}