Find Smallest Common Element in All Rows in C++

Suppose we have a matrix mat where every row is sorted in non-decreasing order, we have to find the smallest common element in all rows. If there is no common element, then return -1. So if the matrix is like −

The output will be 5

To solve this, we will follow these steps −

• Define a map m, n := row count of matrix,

• if n is not 0, then x = column size, otherwise 0

• for i in range 0 to n – 1

  • for j in range 0 to x – 1

    • if m[mat[i, j]] + 1 = i + 1, then increase m[mat[i, j]] by 1

• for each key-value pair i

  • if value of i is n, then return the key of i

• return -1

Example (C++)

Let us see the following implementation to get a better understanding −

 Live Demo

#include <bits/stdc++.h>
using namespace std;
class Solution {
public:
   int smallestCommonElement(vector<vector<int>>& mat) {
      map <int, int> m;
      int n = mat.size();
      int x = n? mat[0].size() : 0;
      for(int i = 0; i < n; i++){
         for(int j = 0; j < x; j++){
            if(m[mat[i][j]] + 1 == i + 1){
               m[mat[i][j]]++;
            }
         }
      }
      map <int, int> :: iterator it = m.begin();
      while(it != m.end()){
         if(it->second == n){
            return it->first;
         }
         it++;
      }
      return -1;
   }
};
main(){
   vector<vector<int>> v = {{1,2,3,4,5},{2,4,5,8,10},{3,5,7,9,11},{1,3,5,7,9}};
   Solution ob;
   cout << (ob.smallestCommonElement(v));
}

Input

[[1,2,3,4,5],[2,4,5,8,10],[3,5,7,9,11],[1,3,5,7,9]]

Output

5