Trie URAL 7192 Chip Factory (15长春J)

题目传送门

题意：从n个数中选出不同的三个数a b c，使得（a+b）^c 最大

分析：先将所有数字按位插入到字典树上，然后删除两个数字，贪心询问与剩下的数字最大异或值。

/************************************************
* Author        :Running_Time
* Created Time  :2015/11/1 14:58:49
* File Name     :J.cpp
 ************************************************/

#include <cstdio>
#include <algorithm>
#include <iostream>
#include <cstring>
#include <cmath>
#include <string>
#include <vector>
#include <queue>
#include <deque>
#include <stack>
#include <list>
#include <map>
#include <set>
#include <bitset>
#include <cstdlib>
#include <ctime>
using namespace std;

#define lson l, mid, rt << 1
#define rson mid + 1, r, rt << 1 | 1
typedef long long ll;
const int N = 1e3 + 10;
const int INF = 0x3f3f3f3f;
const int MOD = 1e9 + 7;
struct Trie {
    int ch[N*30][2], sz;
    int cnt[N*30];
    void init(void) {
        sz = 1; ch[0][0] = ch[0][1] = 0;
        memset (cnt, 0, sizeof (cnt));
    }
    void insert(int x)  {
        int u = 0;
        for (int c, i=30; i>=0; --i)   {
            c = x & (1 << i) ? 1 : 0;
            if (!ch[u][c])  {
                ch[sz][0] = ch[sz][1] = 0;
                ch[u][c] = sz++;
            }
            u = ch[u][c];
            cnt[u]++;
        }
    }
    void remove(int x)  {
        int u = 0;
        for (int c, i=30; i>=0; --i)    {
            c = x & (1 << i) ? 1 : 0;
            u = ch[u][c];
            cnt[u]--;
        }
    }
    int query(int x)    {
        int u = 0;
        for (int c, i=30; i>=0; --i)   {
            c = x & (1 << i) ? 1 : 0;
            if (c == 1) {
                if (ch[u][0] && cnt[ch[u][0]])  u = ch[u][0];
                else    u = ch[u][1], x ^= (1 << i);
            }
            else    {
                if (ch[u][1] && cnt[ch[u][1]])  u = ch[u][1], x ^= (1 << i);
                else    u = ch[u][0];
            }
        }
        return x;
    }
}trie;
int a[N];

int main(void)    {
    int T;  scanf ("%d", &T);
    while (T--) {
        int n;  scanf ("%d", &n);
        for (int i=1; i<=n; ++i)    {
            scanf ("%d", &a[i]);
        }
        int ans = 0;
        trie.init ();
        for (int i=1; i<=n; ++i)    {
            trie.insert (a[i]);
        }
        for (int i=1; i<=n; ++i)    {
            trie.remove (a[i]);
            for (int j=i+1; j<=n; ++j)  {
                trie.remove (a[j]);
                ans = max (ans, trie.query (a[i] + a[j]));
                trie.insert (a[j]);
            }
            trie.insert (a[i]);
        }
        printf ("%d\n", ans);
    }

    return 0;
}