poj 2750 Potted Flower

http://poj.org/problem?id=2750

这道题给出的是个环，如果是一个序列，用线段树很容易做出来，给的是个环就要考虑断点是不是在最大连续和之内。

 1 #include <cstdio>

 2 #include <cstring>

 3 #include <algorithm>

 4 #define maxn 2000000

 5 using namespace std;

 6 

 7 int n,m,x,y;

 8 int a[maxn];

 9 struct node

10 {

11     int l,r;

12     int lmax,rmax,summax;

13     int lmin,rmin,summin;

14     int sum;

15 }tree[maxn*4];

16 

17 

18 void up(int i)

19 {

20     tree[i].sum=tree[i<<1].sum+tree[i<<1|1].sum;

21     tree[i].lmax=max(tree[i<<1].lmax,tree[i<<1].sum+tree[i<<1|1].lmax);

22     tree[i].rmax=max(tree[i<<1|1].rmax,tree[i<<1|1].sum+tree[i<<1].rmax);

23     tree[i].summax=max(max(tree[i<<1].summax,tree[i<<1|1].summax),tree[i<<1].rmax+tree[i<<1|1].lmax);

24 

25     tree[i].lmin=min(tree[i<<1].lmin,tree[i<<1].sum+tree[i<<1|1].lmin);

26     tree[i].rmin=min(tree[i<<1|1].rmin,tree[i<<1|1].sum+tree[i<<1].rmin);

27     tree[i].summin=min(min(tree[i<<1].summin,tree[i<<1|1].summin),tree[i<<1].rmin+tree[i<<1|1].lmin);

28 }

29 void build(int i,int l,int r)

30 {

31     tree[i].l=l;

32     tree[i].r=r;

33     if(l==r)

34     {

35         tree[i].sum=a[l];

36         tree[i].lmax=tree[i].rmax=tree[i].summax=a[l];

37         tree[i].lmin=tree[i].rmin=tree[i].summin=a[l];

38         return ;

39     }

40     int mid=(l+r)>>1;

41     build(i<<1,l,mid);

42     build(i<<1|1,mid+1,r);

43     up(i);

44 }

45 

46 void search1(int i,int l,int r,int key)

47 {

48     if(tree[i].l==l&&tree[i].r==r)

49     {

50          tree[i].sum=key;

51          tree[i].lmax=tree[i].rmax=tree[i].summax=key;

52          tree[i].lmin=tree[i].rmin=tree[i].summin=key;

53          return ;

54     }

55     int mid=(tree[i].l+tree[i].r)>>1;

56     if(r<=mid)

57     {

58         search1(i<<1,l,r,key);

59     }

60     else

61       search1(i<<1|1,l,r,key);

62     up(i);

63 }

64 

65 

66 int main()

67 {

68    while(scanf("%d",&n)!=EOF)

69    {

70        for(int i=1; i<=n; i++)

71        {

72            scanf("%d",&a[i]);

73        }

74        scanf("%d",&m);

75        build(1,1,n);

76        for(int i=0; i<m; i++)

77        {

78            scanf("%d%d",&x,&y);

79            search1(1,x,x,y);

80            if(tree[1].sum==tree[1].summax)

81            {

82                printf("%d\n",tree[1].sum-tree[1].summin);

83            }

84            else

85            printf("%d\n",max(tree[1].summax,tree[1].sum-tree[1].summin));

86        }

87    }

88    return 0;

89 }

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