【POJ 1330】 Nearest Common Ancestors

【题目链接】

           点击打开链接

【算法】

         倍增法求最近公共祖先

【代码】

         

#include <algorithm>
#include <bitset>
#include <cctype>
#include <cerrno>
#include <clocale>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <deque>
#include <exception>
#include <fstream>
#include <functional>
#include <limits>
#include <list>
#include <map>
#include <iomanip>
#include <ios>
#include <iosfwd>
#include <iostream>
#include <istream>
#include <ostream>
#include <queue>
#include <set>
#include <sstream>
#include <stdexcept>
#include <streambuf>
#include <string>
#include <utility>
#include <vector>
#include <cwchar>
#include <cwctype>
#include <stack>
#include <limits.h>
using namespace std;
#define MAXN 10010
#define MAXLOG 20

struct Edge
{
        int to,nxt;
} e[MAXN];

int i,T,tot,n,root,x,y;
int fa[MAXN],dep[MAXN],head[MAXN],anc[MAXN][MAXLOG];

template <typename T> inline void read(T &x)
{
    int f = 1; x = 0;
    char c = getchar();
    for (; !isdigit(c); c = getchar()) { if (c == '-') f = -f; }
    for (; isdigit(c); c = getchar()) x = (x << 3) + (x << 1) + c - '0';
    x *= f;
}
template <typename T> inline void write(T x)
{
    if (x < 0)
    {
        putchar('-');
        x = -x;
    }
    if (x > 9) write(x/10);
    putchar(x%10+'0');
}
template <typename T> inline void writeln(T x)
{
    write(x);
    puts("");
}
inline void add(int x,int y)
{
        tot++;
        e[tot] = (Edge){y,head[x]};
        head[x] = tot;
}
inline void lca_init(int u)
{
        int i,v;
        anc[u][0] = fa[u];
        for (i = 1; i < MAXLOG; i++)
        {
                if (dep[u] <= (i << 1)) break;
                anc[u][i] = anc[anc[u][i-1]][i-1];
        }
        for (i = head[u]; i; i = e[i].nxt)
        {
                v = e[i].to;
                dep[v] = dep[u] + 1;
                lca_init(v);
        }
}
inline int lca(int u,int v)
{
        int i,k;
        if (dep[u] > dep[v]) swap(u,v);
        k = dep[v] - dep[u];
        for (i = 0; i < MAXLOG; i++)    
        {
                if (k & (1 << i))
                        v = anc[v][i];
        }
        if (u == v) return u;
        for (i = MAXLOG - 1; i >= 0; i--)
        {
                if (anc[u][i] != anc[v][i])
                {
                        u = anc[u][i];
                        v = anc[v][i];
                }
        }
        return fa[u];
}

int main() {
        
        read(T);
        while (T--)
        {
                memset(fa,0,sizeof(fa));
                memset(anc,0,sizeof(anc));
                read(n);
                tot = 0;
                for (i = 1; i <= n; i++) head[i] = 0;
                for (i = 1; i < n; i++)
                {
                        read(x); read(y);
                        add(x,y);
                        fa[y] = x;
                }
                for (i = 1; i <= n; i++)
                {
                        if (!fa[i])
                                root = i;
                }
                dep[root] = 1;
                lca_init(root);
                read(x); read(y);
                writeln(lca(x,y));
        }
        return 0;
    
}