1954: Pku3764 The xor-longest Path 字典树 贪心

显然， （a，b）两点间的路径异或和等于 (1,a) 和 (1,b) 两条路径的异或和。 所以一遍dfs就可得出所有的点的异或和。

然后…………字典树上贪心乱搞了。

 1 #include<cstdio>
 2 #include<iostream>
 3 #define rep(i,j,k) for(register int i = j; i <= k; i++)
 4 #define dow(i,j,k) for(register int i = j; i >= k; i--)
 5 #define maxn 100005
 6 using namespace std; 
 7 
 8 inline int read() {
 9     int s = 0, t = 1; char c = getchar();
10     while( !isdigit(c) ) { if( c == '-' ) t = -1; c = getchar(); }
11     while( isdigit(c) ) s = s * 10 + c - 48, c = getchar();
12     return s * t;
13 }
14 
15 struct edge{ int to, v; edge*next; } e[maxn<<1], *pt = e, *head[maxn];
16 inline void add(int x,int y,int v) {
17     pt->to = y, pt->next = head[x], pt->v = v, head[x] = pt++;
18     pt->to = x, pt->next = head[y], pt->v = v, head[y] = pt++;
19 }
20 
21 #define ez(i,j) for(edge*i = head[j]; i; i=i->next)
22 #define to i->to
23 int val[maxn];
24 inline void dfs(int x,int fa) {
25     ez(i,x) if( to != fa ) val[to] = val[x] ^ i->v, dfs(to,x);
26 }
27 
28 int ans = 0, bin[32], all[32];
29 int tot = 0, ch[3000000][2];
30 inline void insert(int v) {
31     int p = 0; 
32     dow(i,30,0)
33         if( v & bin[i] ) {
34             if( ch[p][1] ) p = ch[p][1]; else p = ch[p][1] = ++tot;
35         } else {
36             if( ch[p][0] ) p = ch[p][0]; else p = ch[p][0] = ++tot;
37         }
38 }
39 
40 inline void query(int v) {
41     int p = 0, now = 0;
42     dow(i,30,0) {
43         if( v & bin[i] ) {
44             if( ch[p][0] ) now += bin[i], p = ch[p][0]; else p = ch[p][1];
45         } else {
46             if( ch[p][1] ) now += bin[i], p = ch[p][1]; else p = ch[p][0];
47         } 
48         if( now + all[i-1] <= ans ) return;
49     } ans = max(ans,now);
50 }
51 
52 int main() {
53     bin[0] = 1; rep(i,1,30) bin[i] = bin[i-1] << 1;
54     all[0] = 1; rep(i,1,30) all[i] = all[i-1] | bin[i];
55     int x, y, n = read();
56     rep(i,1,n-1) x = read(), y = read(), add(x,y,read());
57     dfs(1,0);
58     rep(i,1,n) insert(val[i]);
59     rep(i,1,n) query(val[i]);
60     cout<<ans<<endl;
61     return 0;
62 }

 

posted on 2016-05-17 19:42  83131  阅读(187)  评论(0)  编辑  收藏  举报

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