POJ-2985（树状数组 + 并查集 + 二分）

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题目：http://poj.org/problem?id=2985

这题考察了两个点呢，组合并用到了并查集，求第K大的数用到了树状数组，刚开始WA了几次，发现是在组合并之后仅仅update(*, -1)了一次，因为是删除了两个组增加了一个组，应该是update(*, -1)两次，不细心啊。。。

#include <cstdio>#include <cstring>#include <algorithm>using namespace std;#define MAX200005int N, M, G;int father[MAX] = {0};//father[i]表示i所在group的首脑int member[MAX] = {0};//member[i]表示以i为首的group有多少个memberint group[MAX] = {0};//group[i]表示成员有[i-lowbit(i)+1, i]个的group有多少个,inline int lowbit(int x){ return x & -x; }void update(int x, int v){for(; x <= N; x += lowbit(x)){group[x] += v;}}int sum(int x){int tot = 0;for(; x; x -= lowbit(x)) tot += group[x];return tot;}int query(int k){int l = 0, r = N;while(l + 1 < r){int m = (l + r) >> 1;if(sum(m) >= k) r = m;else l = m;}return r;}int find(int x){return x != father[x] ? father[x] = find(father[x]) : x;}void join(int x, int y){int fx = find(x), fy = find(y);if(fx == fy) return;if(member[fx] > member[fy]) swap(fx, fy);father[fx] =  fy;update(member[fy], -1);member[fy] += member[fx];update(member[fx], -1);member[fx] = 0;update(member[fy], 1);--G;}void init(){for(int i = 1; i <= N; ++i) father[i] = i;for(int i = 1; i <= N; ++i) member[i] = 1;G = N;memset(group + 1, 0, N * sizeof(int));for(int i = 0; (1 << i) <= N; ++i) group[1 << i] = N;}int main(){int c, x, y;while(~scanf("%d%d", &N, &M)){init();while(M--){scanf("%d%d", &c, &x);if(c) printf("%d\n", query(G - x + 1));else{scanf("%d", &y);join(x, y);}}}return 0;}

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