new file: P4168.cpp

P4168.cpp

@@ -0,0 +1,144 @@
+#include <bits/stdc++.h>
+using namespace std;
+
+const int MAXN = 40005;
+const int MAX2 = 205;
+
+int n, q;
+int a[MAXN], comp[MAXN], vals[MAXN], vcnt, l0;
+int pcnt[MAX2][MAXN]; // 前缀计数 pcnt[i][v] = 块[0..i]中压缩值v的出现次数
+int bval[MAX2][MAX2]; // 块区间[i,j]的众数值
+int bcnt[MAX2][MAX2]; // 块区间[i,j]的众数出现次数
+
+void lsh() // 离散化
+{
+    for (int i = 0; i < n; i++)
+        vals[i] = a[i];
+    sort(vals, vals + n);
+    vcnt = unique(vals, vals + n) - vals;
+    for (int i = 0; i < n; i++)
+        comp[i] = lower_bound(vals, vals + vcnt, a[i]) - vals;
+}
+
+void bkcnt()
+{
+    int totalBlocks = n / l0;
+    memset(pcnt, 0, sizeof(pcnt));
+    for (int i = 0; i <= totalBlocks; i++)
+    {
+        if (i > 0)
+            memcpy(pcnt[i], pcnt[i - 1], sizeof(int) * vcnt);
+        for (int j = i * l0; j < (i + 1) * l0 && j < n; j++)
+            pcnt[i][comp[j]]++;
+    }
+}
+
+void pre() // O(n sqrt(n))
+{
+    static int freq[MAXN];
+    int totalBlocks = n / l0;
+    for (int i = 0; i <= totalBlocks; ++i)
+    {
+        int maxc = 0, maxp = 0;
+        for (int j = i; j <= totalBlocks; ++j)
+        {
+            for (int k = j * l0; k < (j + 1) * l0 && k < n; ++k)
+            {
+                freq[comp[k]]++;
+                int f = freq[comp[k]];
+                if (f > maxc || (f == maxc && a[k] < a[maxp]))
+                    maxc = f, maxp = k;
+            }
+            bval[i][j] = a[maxp];
+            bcnt[i][j] = maxc;
+        }
+        memset(freq, 0, sizeof(int) * vcnt);
+    }
+}
+
+int query(int l, int r) // 返回众数 O(sqrt(n))
+{
+    static int freq[MAXN];
+    static int sideVals[MAXN];
+    int sideN;
+
+    if (l / l0 == r / l0) // 在同一个区块内
+    {
+        int maxc = 0, maxp = l;
+        for (int i = l; i <= r; ++i)
+        {
+            freq[comp[i]]++;
+            int f = freq[comp[i]];
+            if (f > maxc || (f == maxc && a[i] < a[maxp]))
+                maxc = f, maxp = i;
+        }
+        int result = a[maxp];
+        for (int i = l; i <= r; ++i)
+            freq[comp[i]] = 0;
+        return result;
+    }
+    int bl = l / l0, br = r / l0;
+    // 中间完整块 [bl+1, br-1] 的众数
+    int best_val = 0, best_cnt = 0;
+    if (bl + 1 <= br - 1)
+    {
+        best_val = bval[bl + 1][br - 1];
+        best_cnt = bcnt[bl + 1][br - 1];
+    }
+    // 两侧零散部分 — 用数组O(1)计数
+    sideN = 0;
+    for (int i = l; i <= (bl + 1) * l0 - 1; ++i)
+    {
+        int cv = comp[i];
+        freq[cv]++;
+        if (freq[cv] == 1)
+            sideVals[sideN++] = cv;
+    }
+    for (int i = br * l0; i <= r; ++i)
+    {
+        int cv = comp[i];
+        freq[cv]++;
+        if (freq[cv] == 1)
+            sideVals[sideN++] = cv;
+    }
+    for (int j = 0; j < sideN; ++j)
+    {
+        int cv = sideVals[j];
+        int mid = 0;
+        if (bl + 1 <= br - 1)
+            mid = pcnt[br - 1][cv] - pcnt[bl][cv];
+        int total = freq[cv] + mid;
+        int ov = vals[cv];
+        if (best_cnt == 0 || total > best_cnt || (total == best_cnt && ov < best_val))
+            best_cnt = total, best_val = ov;
+    }
+    // 清理
+    for (int j = 0; j < sideN; ++j)
+        freq[sideVals[j]] = 0;
+    return best_val;
+}
+
+int main()
+{
+    // freopen("dandelion.in", "r", stdin);
+    // freopen("dandelion.out", "w", stdout);
+    scanf("%d%d", &n, &q);
+    l0 = (int)sqrt(n);
+    for (int i = 0; i < n; ++i)
+        scanf("%d", &a[i]);
+    lsh();
+    bkcnt();
+    pre();
+    int ans = 0;
+    while (q--)
+    {
+        int l, r;
+        scanf("%d %d", &l, &r);
+        l = ((l + ans - 1) % n) + 1, r = ((r + ans - 1) % n) + 1; // 加密强制在线算法
+        if (l > r)
+            swap(l, r);
+        ans = query(l - 1, r - 1);
+        printf("%d\n", ans);
+    }
+    return 0;
+}