Create solution.cpp

Author: samarthswami1016

solution/3400-3499/3470.Permutations IV/solution.cpp

@@ -0,0 +1,40 @@
+class Solution {
+    long long f[105];
+public:
+    vector<int> permute(int n, long long k) {
+        int i,j;
+        for(i=f[0]=1;i<=n;i++)if(f[i-1]>=k)f[i]=k;
+        else f[i]=f[i-1]*(i+1>>1);
+        if(!(n&1))f[n]*=2;
+        if(f[n]<k)return {};
+        k--;
+        vector<int> ans(n),a[2];
+        for(i=0;i<n;i++)a[i&1].push_back(i);
+        if(n&1)
+        {
+            ans[0]=k/f[n-1]*2;
+            k-=ans[0]/2*f[n-1];
+        }
+        else
+        {
+            ans[0]=k/f[n-1];
+            k-=ans[0]*f[n-1];
+        }
+        a[ans[0]&1].erase(lower_bound(a[ans[0]&1].begin(),a[ans[0]&1].end(),ans[0]));
+        for(i=1;i<n;i++)
+        {
+            if(n&1)
+            {
+                ans[i]=a[i&1][k/f[n-i-1]];
+            }
+            else
+            {
+                ans[i]=a[(ans[0]^i)&1][k/f[n-i-1]];
+            }
+            k%=f[n-i-1];
+            a[ans[i]&1].erase(lower_bound(a[ans[i]&1].begin(),a[ans[i]&1].end(),ans[i]));
+        }
+        for(i=0;i<n;i++)ans[i]++;
+        return ans;
+    }
+};