牛客-XOR TREE

题目传送门

sol:假如把这题树上这个条件改成序列，那么对于操作2，求某区间$[l, r]$所有连续子序列的异或和。观察得出若$r - l + 1$是偶数，则答案是$[l, r]$区间的异或和。否则，答案是$[l, r]$区间内下标奇偶性与$l$不同的元素的异或和。那么可以用三颗线段树来分别维护奇数下标、偶数下标、所有下标的异或和。至于在树上完成操作，树剖解决。

• 树剖 + 线段树

  #include <bits/stdc++.h>
  using namespace std;
  typedef long long LL;
  typedef pair<int, int> PII;
  const int MAXN = 2e5 + 10;
  inline int read() {
      int n = 0; char c = getchar();
      while (c < '0' || c > '9') c = getchar();
      while (c >= '0' && c <= '9') {
          n = 10 * n + (c ^ '0');
          c = getchar();
      }
      return n;
  }
    
  struct {int v, n;} edge[2 * MAXN];
  int val[MAXN], head[MAXN];
  void add_edge(int u, int v) {
      static int cnt = 1;
      edge[cnt].v = v;
      edge[cnt].n = head[u];
      head[u] = cnt++;
  }
  // 树剖
  int fat[MAXN], son[MAXN], siz[MAXN], dep[MAXN];
  int top[MAXN], rnk[MAXN], ord[MAXN];
  void dfs(int u, int f, int d) {
      fat[u] = f, dep[u] = d, siz[u] = 1;
      for (int i = head[u]; i != -1; i = edge[i].n) {
          int v = edge[i].v;
          if (v == fat[u]) continue;
          dfs(v, u, d + 1);
          siz[u] += siz[v];
          if (siz[v] > siz[son[u]]) {
              son[u] = v;
          }
      }
  }
  void dfs(int u, int t) {
      static int cnt = 1;
      top[u] = t, ord[u] = cnt, rnk[cnt++] = u;
      if (!son[u]) return;
      dfs(son[u], t);
      for (int i = head[u]; i != -1; i = edge[i].n) {
          int v = edge[i].v;
          if (v == fat[u] || v == son[u]) continue;
          dfs(v, v);
      }
  }
    
  // 线段树
  struct SegTree {
      struct Node {
          int l, r;
          int val;
      } node[4 * MAXN];
      void build(int i, int l, int r) {
          node[i].l = l, node[i].r = r;
          node[i].val = 0;
          if (l == r) return;
          int mid = l + r >> 1;
          build(i << 1, l, mid);
          build(i << 1 | 1, mid + 1, r);
      }
      void update(int i, int ind, int val) {
          if (node[i].l == node[i].r) {
              node[i].val = val;
              return;
          }
          int mid = node[i].l + node[i].r >> 1;
          if (ind <= mid) update(i << 1, ind, val);
          else update(i << 1 | 1, ind, val);
          node[i].val = node[i << 1].val ^ node[i << 1 | 1].val;
      }
      int query(int i, int l, int r) {
          if (node[i].l == l && node[i].r == r) return node[i].val;
          int mid = node[i].l + node[i].r >> 1;
          if (r <= mid) return query(i << 1, l, r);
          if (l > mid) return query(i << 1 | 1, l, r);
          return query(i << 1, l, mid) ^ query(i << 1 | 1, mid + 1, r);
      }
  } tree[3];
  // 求路径和
  int query(int u, int v) {
      int val = 0, tag;
      int tu = top[u], tv = top[v];
      if ((dep[u] & 1) != (dep[v] & 1)) tag = 2;
      else tag = !(dep[u] & 1);
      while (tu != tv) {
          if (dep[tu] >= dep[tv]) { // 跳tu到u
              val ^= tree[tag].query(1, ord[tu], ord[u]);
              u = fat[tu], tu = top[u];
          } else {
              val ^= tree[tag].query(1, ord[tv], ord[v]);
              v = fat[tv], tv = top[v];
          }
      }
      if (dep[u] <= dep[v]) {
          val ^= tree[tag].query(1, ord[u], ord[v]);
      } else {
          val ^= tree[tag].query(1, ord[v], ord[u]);
      }
      return val;
  }
  // 主函数
  int main() {
      int n = read(), m = read();
      for (int i = 1; i <= n; i++) val[i] = read();
      memset(head, -1, sizeof(head));
      for (int i = 2; i <= n; i++) {
          int u = read(), v = read();
          add_edge(u, v);
          add_edge(v, u);
      }
      dfs(1, -1, 1);
      dfs(1, 1);
      tree[0].build(1, 1, n), tree[1].build(1, 1, n), tree[2].build(1, 1, n);
      for (int i = 1; i <= n; i++) {
          tree[dep[i] & 1].update(1, ord[i], val[i]);
          tree[2].update(1, ord[i], val[i]);
      }
      for (int i = 1; i <= m; i++) {
          int op = read();
          if (op == 1) {
              int ind = read(), val = read();
              tree[dep[ind] & 1].update(1, ord[ind], val);
              tree[2].update(1, ord[ind], val);
          } else {
              int u = read(), v = read();
              printf("%d
  ", query(u, v));
          }
      }
      return 0;
  }

  因为这道题专门去学了树剖，这题的数据真的是很玄学啊。我一样的代码第一次提交过80%数据超时，之后提交就能AC。另外一份感觉复杂度一样的代码一直过80%数据超时。最后20%数据有毒。