接金币这个c++代码理解不了，求解释

原题链接：http://exercise.acmcoder.com/online/online_judge_ques?ques_id=3009&konwledgeId=134
#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

#define INF 0x3f3f3f3f

typedef pair<int, int> p;

bool compare(p p1,p p2){
	return p1.second < p2.second;
}

int getcoin(vector<p> vec, int pos, int time){
	int res = 0;
	for (int i = 0; i < vec.size(); i++){
		if (vec[i].first == pos&&vec[i].second == time){
			res++;
		}
		if (vec[i].second > time)
			break;
	}
	return res;
}

int main(){
	int n;
	cin >> n;
	vector<p> vec;
	for (int i = 0; i < n; i++){
		p p1;
		cin >> p1.first >> p1.second;
		vec.push_back(p1);
	}
	int dp[11][2];
	for (int i = 0; i <= 10; i++){
		for (int j = 0; j < 2; j++)
			dp[i][j] =-INF;
	}
	dp[5][0] = 0;
	sort(vec.begin(), vec.end(), compare);
	int T = vec[vec.size() - 1].second;
	for (int t = 0; t <=T; t++)
		for (int j = 0; j <= 10; j++){
			for (int k = -1; k <= 1; k++){
				if (dp[j][t%2] == -INF)break;
				else{
					dp[j + k][(t + 1) % 2] = max(dp[j + k][(t + 1) % 2],dp[j][t%2]+getcoin(vec,j+k,t

+1));//这行理解不了啊
				}
			}
		}
	int ans = 0;
	for (int i = 0; i <= 10; i++){
		ans = ans>dp[i][T % 2] ? ans : dp[i][T % 2];
	}
	cout << ans;
	return 0;
}