478D - Red-Green Towers - CodeForces Solution

#include <bits/stdc++.h>
#define pb push_back
#define ll long long
using namespace std;
int dp[2][200005];
int mod=1e9+7;
int main()
{
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    int r,g;
    cin>>r>>g;
    int sum=r+g;
    int h=0;
    while(h*(h+1)/2<=sum)h++;
    h--;
    int rr=r;
    int gg=g;
    if(r>0)dp[1][1]=1;
    if(g>0)dp[1][0]=1;
    int roling=0;
    for(int i=2;i<=h;i++){
        int temp=(i*(i-1))/2;
        for(int j=0;j<=rr;j++){
            int g=gg-temp+j;
            dp[i&1][j]=0;
            if(g<0)continue;
            if(g>=i){
                dp[i&1][j]+=dp[i&1^1][j];
                dp[i&1][j]%=mod;
            }
            if(j>=i){
                dp[i&1][j]+=dp[i&1^1][j-i];
                dp[i&1][j]%=mod;
            }
//            cout<<dp[i&1][j]<<" ";
        }
//        cout<<endl;
    roling=roling^1;
    }
    int ans=0;
    for(int i=0;i<=rr;i++){
        ans+=dp[roling^1][i];
        ans%=mod;
    }
    cout<<ans;
    return 0;
}