习题7-13(uva-817)

#include <iostream>
#include <istream>
#include <sstream>
#include <vector>
#include <stack>
#include <list>
#include <map>
#include <set>
#include <deque>
#include <queue>
#include <cstring>
#include <unordered_map>
#include <unordered_set>
#include <algorithm>
#include <numeric>
#include <chrono>
#include <ctime>
#include <cmath>
#include <cctype>
#include <string>
#include <cstdio>
#include <iomanip>


#include <thread>
#include <mutex>
#include <condition_variable>
#include <functional>
#include <iterator>
using namespace std;
const int res = 2000;
int n;
string input, str, temp;
vector<string> ans;
//字典序优先级
char op[3] = { '*', '+', '-' };

int GetResult(int a, int b, int o) {
	if (o == '+') return a + b;
	else if (o == '-') return a - b;
	return a * b;
}

void Check(string& t) {
	stack<int> st;
	stack<char> sign;

	int before = 0, last = 0, isAdd = -1, o;
	for (int i = 0; i < t.size(); i++) {
		if (isdigit(t[i])) {
			last = last * 10 + (t[i] - '0');
		}
		else {
			if (!sign.empty() && sign.top() == '*') {
				before = st.top(); st.pop();
				o = sign.top(); sign.pop();
				last = GetResult(before, last, o);
			}
			if (t[i] == '+' || t[i] == '-') {
				while (!sign.empty()) {
					before = st.top(); st.pop();
					o = sign.top(); sign.pop();
					last = GetResult(before, last, o);
				}
			}
			st.push(last);
			sign.push(t[i]);
			last = 0;
		}
	}
	while (!sign.empty()) {
		before = st.top(); st.pop();
		o = sign.top(); sign.pop();
		last = GetResult(before, last, o);
	}
	if (last == res) {
		ans.push_back(t);
	}
}

void dfs(int pos, string& cur, string& t) {

	if (pos >= n) {
		if (t.empty()) return;
		if (!cur.empty()) t += cur;
		Check(t);
		if (!cur.empty()) t.erase(t.size() - cur.size());
		return;
	}

	char c = input[pos];
	if (cur != "0") {
		//拼接数字
		cur.push_back(c);
		dfs(pos + 1, cur, t);
		cur.pop_back();
	}
	
	if (!cur.empty()) {
		//数字拼接完成并添加运算符号
		t += cur;
		string tt(1, c);
		for (int i = 0; i < 3; i++) {
			t.push_back(op[i]);
			dfs(pos + 1, tt, t);
			t.pop_back();
		}
		t.erase(t.size() - cur.size());
	}
}

int main()
{
	int kcase = 0;

	while (cin >> input && input != "=") {
		temp.clear();
		ans.clear();
		str = "";
		n = input.size() - 1;
		dfs(0, str, temp);
		cout << "Problem " << ++kcase << endl;
		if (ans.empty()) cout << "  IMPOSSIBLE" << endl;
		else {
			for (auto it = ans.begin(); it != ans.end(); it++)
			{
				cout << "  " << *it << "=" << endl;
			}
		}
	}
	return 0;
}