## Thursday, December 18, 2014

### Codeforces Round #283 Div. 2 Problem D - Tennis Game

Problem Statement:
496D - Tennis Game

Solution:
Another problem where harmonic series comes into the picture! Just realized on how to solve this problem after the contest finished :P

Firstly build 2 arrays of cumulative scores of both players. Then we try every possible t from 1 to N:
1. We search for all positions in which the set ends using binary search several times. At any junction, if we cannot find the desired cumulative scores in which a set can end, it means that t is not valid.
2. Keep track on the scores of of the player, and who scored the last point.
3. t is valid iff the scores are not equal, and the last player who scored the last point is also the one who have higher score.

For each t, the worst case running time will be $$O(\frac{N}{t} \lg{N})$$. In total, we will have a worst case running time of $$N\lg{N}\sum_{t=1}^{t=N} \frac{N}{t} = O(N\lg^2{N})$$ since harmonic series is bounded logarithmically. :D (solving this problem is quite satisfying :P)

Implementation:

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <vector>
#include <utility>
using namespace std;

int sum[100003];
int _sum[100003];
int a[100003];
vector<pair<int,int> > st;
int N;

int bsearch(int * s, int val, int margin){
int lo = 0, hi = N-1, mid;
while(lo <= hi){
mid = (lo+hi)/2;
if(s[mid] - margin < val){
lo = mid + 1;
} else {
hi = mid - 1;
}
}
return lo;
}

int main(){
scanf("%d", &N);
for(int i=0;i<N;++i){
scanf("%d", &a[i]);
a[i]--;
sum[i] = a[i];
if(i>0) sum[i] += sum[i-1];
_sum[i] = i + 1 - sum[i];
}
for(int k=1;k<=N;++k){
bool ok = true;
int score[2] = {0,0};
int acc[2] = {0,0};
int last_win = -1;
while(1){
int pos = bsearch(sum, k, acc[0]);
if(pos >= N || sum[pos]-acc[0] != k || sum[pos] - acc[0] <= _sum[pos] - acc[1]) {
pos = bsearch(_sum, k, acc[1]);
if(pos >= N || _sum[pos]-acc[1] != k) {
ok = false;
break;
} else {
++score[1];
last_win = 1;
}
} else {
score[0] ++;
last_win = 0;
}
acc[0] = sum[pos];
acc[1] = _sum[pos];
if(pos == N-1) break;
}
if(!ok || score[0] == score[1]) continue;
if((score[0] > score[1] && last_win == 0) ||
(score[0] < score[1] && last_win == 1) ){
st.push_back(make_pair(max(score[0], score[1]), k));
}
}
sort(st.begin(), st.end());
printf("%d\n", (int)st.size());
for(int i=0;i<st.size();++i){
printf("%d %d\n", st[i].first, st[i].second);
}
return 0;
}