1.砝码称重（java）
CSDN上已经有人写出来过，想写一下思路
//方法2：DFS搜索
package com.liu.ex1;
import java.util.ArrayList;
import java.util.Scanner;

public class Main1 {
public static int[] value = {1,3,9,27,81};
public static int n = 0;
public static ArrayList result = new ArrayList();

public int getSum() {
    int sum = 0;
    for(int i = 0;i < result.size();i++)
        sum += result.get(i);
    return sum;
}

public void dfs(int step) {
    if(step == 5) {
        if(getSum() == n) {
            int i = result.size() - 1;
            for(;i >= 0;i--) {
                if(result.get(i) != 0) {
                    System.out.print(result.get(i));
                    i--;
                    break;
                }
            }
            for(;i >= 0;i--) {
                if(result.get(i) == 0)
                    continue;
                if(result.get(i) > 0)
                    System.out.print("+");
                System.out.print(result.get(i));
            }
        }
    } else {
        for(int i = -1;i <= 1;i++) {
            int a = i * value[step];
            result.add(a);
            dfs(step + 1);
            result.remove(result.size() - 1);
        }
     }
}

public static void main(String[] args) {
    Main1 test = new Main1();
    Scanner in = new Scanner(System.in);
    n = in.nextInt();
    test.dfs(0);
}

}