歸并排序：

#include <cstdio> #define ll long long using namespace std; const int maxn = 500001; ll a[maxn], s[maxn], ans = 0, n;//ans用來記錄逆序對數量 void merge_sort(ll l,ll r) {if(l == r) return ;ll mid = (l + r)>>1;merge_sort(l,mid);merge_sort(mid+1,r);//不斷劃分成兩個數列 ll i = l, j = mid+1, k =l;while(i <= mid&&j <= r){if(a[i]<=a[j]){s[k] = a[i],k++,i++;}else {s[k] = a[j],k++,j++;ans+=mid-i+1; }}while(i <= mid)s[k] = a[i],k++,i++;while(j<=r)s[k] = a[j],k++,j++;for(int i = l; i <= r; i++)a[i] = s[i]; } int main() {scanf("%lld",&n);for(int i = 1; i <= n; i++){scanf("%lld",&a[i]); } merge_sort(1,n);/*for(int i = 1; i <= n; i++){printf("%d ",a[i]);}*/printf("%lld",ans);return 0; }

BIT：

#include <cstdio> #include <cstring> #include <iostream> #include <algorithm> #define ll long long using namespace std; const int maxn = 1e6 + 10; ll n, m, A[maxn], B[maxn], ans, L[maxn]; class BIT{public:ll tree[maxn];void update(ll pos, ll val){while(pos <= n){tree[pos] += val;pos += lowbit(pos);}}ll query(ll pos){ll res = 0;while(pos){res += tree[pos];pos -= lowbit(pos);}return res;}private:ll lowbit(ll x){return x & -x;} }T; int Search(ll x) {return lower_bound(B + 1, B + 1 + m, x) - B; } int main() {scanf("%lld",&n);for(int i = 1; i <= n; i++) scanf("%lld",&A[i]), B[i] = A[i];sort(B + 1, B + 1 + n);m = unique(B + 1, B + 1 + n) - B - 1;for(int i = 1; i <= n; i++) A[i] = Search(A[i]);for(int i = 1; i <= n; i++){T.update(A[i], 1);L[i] = i - T.query(A[i]);}for(int i = 1; i <= n; i++) ans += L[i];printf("%lld",ans);return 0; }

轉載于:https://www.cnblogs.com/MisakaAzusa/p/11019610.html

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