Sort by frequency

Arrays-algos/mountainSubarrayProblem.cpp

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+// https://practice.geeksforgeeks.org/problems/mountain-subarray-problem/1
+
+#include <bits/stdc++.h>
+using namespace std;
+
+class Solution{
+  public:
+    vector<bool> processQueries(int a[], int n, vector<pair<int, int>> &queries,
+                                int q) {
+
+        vector<bool>ans;
+        int left[n], right[n];
+        left[0]=0;
+        right[n-1]=n-1;
+        int k=0;
+
+        // left[i] stores the last index on left side which is increasing 
+        // i.e. greater than its previous element
+        for(int i=1; i<n; i++)
+            {
+                if(a[i-1]<a[i])
+                    k=i;
+                left[i]=k;
+            }
+        // right[i] will store the first index on the right side which is decreasing 
+        // i.e. greater than its next element. 
+        for(int i=n-2; i>=0; i--)  
+        {
+            if(a[i+1]<a[i])
+                k=i;
+            right[i]=k;
+
+        }
+
+        for(auto x: queries)
+        {
+            int l= x.first;
+            int r= x.second;
+
+            if(left[r]<=right[l])
+                ans.push_back(true);
+            else
+                ans.push_back(false);
+        }
+    return ans;
+    }
+};
+
+int main() {
+    int tc;
+    cin >> tc;
+    while (tc--) {
+        int n, i, q;
+        cin >> n;
+        int a[n];
+        for (i = 0; i < n; i++) {
+            cin >> a[i];
+        }
+        cin >> q;
+        vector<pair<int, int>> queries(q);
+        for (i = 0; i < q; i++) {
+            cin >> queries[i].first >> queries[i].second;
+        }
+        Solution obj;
+        auto v = obj.processQueries(a, n, queries, q);
+        for (bool u : v) {
+            cout << (u ? "Yes\n" : "No\n");
+        }
+    }
+    return 0;
+} 

Hashing/sortByFrequency.cpp

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+//https://practice.geeksforgeeks.org/problems/sorting-elements-of-an-array-by-frequency-1587115621/1/
+// Sorting Elements of an Array by Frequency 
+
+#include<bits/stdc++.h>
+using namespace std;
+
+class Solution{
+    public:
+
+    static bool cmp( pair<int, int> &a, pair<int, int> &b)
+    {
+        if(a.second==b.second)
+            return a.first<b.first;
+        return a.second>b.second;
+    }
+
+    vector<int> sortByFreq(int arr[],int n)
+    {
+        //Your code here
+        vector<int>ans;
+        map<int,int> m;
+        vector<pair<int, int> > A;
+
+        for(int i=0; i<n; i++)
+        {
+            int x= arr[i];
+            m[x]++;
+        }
+        // Copy key-value pair from Map
+        // to vector of pairs
+        for (auto &it : m) {
+            A.push_back(it);
+        }
+        sort(A.begin(), A.end(), cmp) ;
+
+        for(auto x: A)
+        {
+           while(x.second)
+           {
+               ans.push_back(x.first);
+               x.second--;
+           }
+
+        }   
+
+        return ans;
+    }
+};
+
+int main() {
+
+
+	int t;
+	cin >> t;
+
+
+	while(t--){
+
+
+	    int n;
+	    cin >> n;
+
+	    int a[n+1];
+
+	    for(int i = 0;i<n;i++){
+	        cin >> a[i];
+	    }
+	    Solution obj;
+	    vector<int> v;
+	    v = obj.sortByFreq(a,n);
+	    for(int i:v)
+	        cout<<i<<" ";
+	    cout << endl;
+	}
+
+	return 0;
+}