递归

1.经典汉诺塔：

//x to z tmp:y import java.io.*; public class Main { public static void hanoi(int n, char x, char y, char z) { if(n != 0) { hanoi(n-1, x, z, y); System.out.println(x + "->" + z); hanoi(n-1, y, x, z); } } public static void main(String[] args) throws IOException { StreamTokenizer in =new StreamTokenizer(new BufferedReader(new InputStreamReader(System.in))); int n; while(in.nextToken() != StreamTokenizer.TT_EOF) { n = (int) in.nval; hanoi(n, 'a', 'b', 'c'); } } }

2.必须经过中间

//x to z by y import java.io.*; public class Main { static int len = 0; public static void hanoi(int n, char x, char y, char z) { if(n != 0) { hanoi(n-1, x, y, z); System.out.println(x + "->" + y); len++; hanoi(n-1, z, y, x); System.out.println(y + "->" + z); hanoi(n-1, x, y, z); len++; } } public static void main(String[] args) throws IOException { StreamTokenizer in =new StreamTokenizer(new BufferedReader(new InputStreamReader(System.in))); int n; while(in.nextToken() != StreamTokenizer.TT_EOF) { n = (int) in.nval; hanoi(n, 'a', 'b', 'c'); System.out.println(len); } } }

3.全排列生成器

import java.io.*; public class Main { public static void perm(int[] a, int k, int n) { if (k == n) { for (int i = 1; i <= n; ++i) { if (i == 1) System.out.print(a[1]); else System.out.print(" " + a[i]); } System.out.println(); } else { for (int i = k; i <= n; ++i) { int tmp = a[k]; a[k] = a[i]; a[i] = tmp; perm(a, k + 1, n); tmp = a[i]; a[i] = a[k]; a[k] = tmp; } } } public static void main(String[] args) throws IOException { StreamTokenizer in = new StreamTokenizer(new BufferedReader( new InputStreamReader(System.in))); while (in.nextToken() != StreamTokenizer.TT_EOF) { int n = (int) in.nval; int[] a = new int[n + 1]; for (int i = 1; i <= n; ++i) a[i] = i; perm(a, 1, n); } } }

4.01背包，回溯

import java.io.*; public class Main { static int[] v, w; static int maxv, c, n; public static void knap(int i, int tw, int tv) { if(i == n) { if(tw<=c && tv>maxv) { maxv = tv; } } else { if(tw+w[i]<=c) knap(i+1, tw+w[i], tv+v[i]); knap(i+1, tw, tv); } } public static void main(String[] args) throws IOException { StreamTokenizer in = new StreamTokenizer(new BufferedReader(new InputStreamReader(System.in))); in.nextToken(); int t= (int) in.nval; while(t-- != 0) { maxv = 0; in.nextToken(); n = (int) in.nval; v = new int[n]; w = new int[n]; in.nextToken(); c = (int) in.nval; for(int i=0; i<n; ++i) { in.nextToken(); v[i] = (int) in.nval; } for(int i=0; i<n; ++i) { in.nextToken(); w[i] = (int) in.nval; } knap(0,0,0); System.out.println(maxv); } } }