'commit'

2025-08-30 18:35:01 +08:00
commit 1f397eca87
6258 changed files with 1487241 additions and 0 deletions
--- a/TangDou/AcWing/P1908_3.cpp
+++ b/TangDou/AcWing/P1908_3.cpp
@@ -0,0 +1,87 @@
+/*
+逆序对数量：
+归并排序解法 https://blog.csdn.net/m0_50299150/article/details/122729602
+树状数组解法 https://blog.csdn.net/m0_50299150/article/details/122729602
+权值线段树解法
+*/
+#include <bits/stdc++.h>
+using namespace std;
+typedef long long LL;
+const int N = 5e5 + 10;
+int a[N];
+vector<int> nums;
+//手写二分版本+离散化 	325 ms
+int n;
+struct node {
+    int l, r, sum;
+} tr[N << 2];
+
+void build(int root, int l, int r) {
+    tr[root] = {l, r, 0};
+    if (l == r) return;
+    int mid = (l + r) >> 1;
+    build(root << 1, l, mid);
+    build(root << 1 | 1, mid + 1, r);
+}
+void pushup(int root) {
+    tr[root].sum = tr[root << 1].sum + tr[root << 1 | 1].sum;
+}
+void modify(int root, int pos, int val) {
+    if (tr[root].l == pos && tr[root].r == pos) {
+        tr[root].sum += val;
+        return;
+    }
+    int mid = tr[root].l + tr[root].r >> 1;
+    if (mid >= pos)
+        modify(root << 1, pos, val);
+    else
+        modify(root << 1 | 1, pos, val);
+    pushup(root);
+}
+int query(int root, int l, int r) {
+    if (tr[root].l == l && tr[root].r == r) return tr[root].sum;
+    int mid = tr[root].l + tr[root].r >> 1;
+    if (mid < l)
+        return query(root << 1 | 1, l, r);
+    else {
+        if (r <= mid)
+            return query(root << 1, l, r);
+        else
+            return query(root << 1, l, mid) + query(root << 1 | 1, mid + 1, r);
+    }
+}
+//改成手写二分查询，AC掉洛谷此题
+int find(int x) {
+    int l = 0, r = nums.size() - 1;
+    while (l < r) {
+        int mid = (l + r) >> 1;
+        if (nums[mid] >= x)
+            r = mid;
+        else
+            l = mid + 1;
+    }
+    return l + 1;
+}
+int main() {
+    //优化读入
+    ios::sync_with_stdio(false);
+    cin >> n;
+    for (int i = 1; i <= n; i++) {
+        cin >> a[i];
+        nums.push_back(a[i]);
+    }
+    //离散化
+    sort(nums.begin(), nums.end());
+    nums.erase(unique(nums.begin(), nums.end()), nums.end());
+
+    LL ans = 0;
+    build(1, 1, nums.size() + 1);
+    //边查边添加,查找它后面比它大的数字个数
+    for (int i = 1; i <= n; i++) {
+        int p = find(a[i]);
+        ans += query(1, p + 1, nums.size() + 1);
+        modify(1, p, 1);
+    }
+    cout << ans << endl;
+    return 0;
+}