穷举所有的可能性，从杯子里面水变化角度来讲，每个状态到下一个状态只有6种行为：3L杯子装满，5L杯子装满，3L倒空，5L倒空，3L倒到5L，5L倒到3L。最终结果如果5L的里面有4L水则得到一个解。如果当前状态和前面一个状态重复，则取消继续往后的探索。

public class test{private static Status[] steps = new Status[100];public static void main(String[] args){steps[0] = new Status();solve(1);}private static void solve(final int depth){//paint(depth-1);if(duplicate(depth - 1)){return;}if(steps[depth - 1].cup5 == 4){paint(depth - 1);return;}Status preStatus = steps[depth - 1];steps[depth] = preStatus.full3();solve(depth + 1);steps[depth] = preStatus.full5();solve(depth + 1);steps[depth] = preStatus.trans3to5();solve(depth + 1);steps[depth] = preStatus.trans5to3();solve(depth + 1);steps[depth] = preStatus.empty3();solve(depth + 1);		steps[depth] = preStatus.empty5();solve(depth + 1);}private static boolean duplicate(int depth){for(int i=0 ; i < depth ; i++){if( steps[i].equals(steps[depth])){return true;}}return false;}private static void paint(int depth){for(int i = 0; i <= depth; i++){System.out.print(steps[i]+" ");}System.out.println();}}class Status{int cup3;int cup5;public Status(){}public Status(Status s){this.cup3 = s.cup3;this.cup5 = s.cup5;}public Status full3(){Status newStatus = new Status(this);newStatus.cup3 = 3;return newStatus;}public Status full5(){Status newStatus = new Status(this);newStatus.cup5 = 5;return newStatus;}public Status trans3to5(){Status newStatus = new Status(this);if(5 - newStatus.cup5 >= newStatus.cup3){		newStatus.cup5 += newStatus.cup3;newStatus.cup3 = 0;}else{newStatus.cup3 -= (5 - newStatus.cup5);newStatus.cup5 = 5;}return newStatus;		}public Status trans5to3(){Status newStatus = new Status(this);if(newStatus.cup5 <= 3 - newStatus.cup3){newStatus.cup3 += newStatus.cup5;		newStatus.cup5 = 0;}else{newStatus.cup5 -= (3 - newStatus.cup3);newStatus.cup3 = 3;}return newStatus;		}public Status empty3(){Status newStatus = new Status(this);newStatus.cup3 = 0;return newStatus;}public Status empty5(){Status newStatus = new Status(this);newStatus.cup5 = 0;return newStatus;}public String toString(){return "["+this.cup3+"#"+this.cup5+"]";}public boolean equals(Object o){if(o == null){return false;}if(o == this){return true;}if(o instanceof Status){Status s = (Status)o;return s.cup3 == this.cup3 && s.cup5 == this.cup5;}return false;}
}