Added English Tutorial for LOJ-1397: Sudoku Solver

1397/en.md

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+# Sudoku Solver
+
+###### Helpful Resources:
+
+[Sudoku Solver Geeks For Geeks](https://www.geeksforgeeks.org/sudoku-backtracking-7/)
+
+[Understanding Sudo solves in CPP](https://www.youtube.com/watch?v=lIOupNAafT0)
+
+[Introduction to Backtracking](https://www.youtube.com/watch?v=DKCbsiDBN6c)
+
+###### Solution Approach
+
+Firstly we implement a basic sudoku solver with five important points:
+
+1)We first have to find the empty positions for that we have the findEmptyLocation function
+
+2) To solve a sudoku we have to place a number at an empty position denoted by '.' in this question
+
+3) We use the isSafe function to determine whether it is valid to place a particular number here
+
+4) If empty location found, we iterate from 1 to 9 and try to place that number in that empty position, checking whether it is safe to place the value in each iteration
+
+5) If the empty positions are such that we can't just place 1 to 9, then we can conclude that the Sudoku is not solvable and the empty positions remain as 0 denoting that it cannot be filled
+
+To summarise,
+
+In each iteration, we find an empty location. At the empty location, we place a value absent from the same row, box and column, and check if the current state of board can be solved. If it cannot be solved, we output so. 
+Otherwise we place the appropriate value and finally print the final state of the sudoku matrix.
+
+
+Note: This is a classic backtracking problem, it does not require dynamic programming approach, it cannot be solved simply by a naive brute force solution.
+
+
+## Solution in C++
+```cpp
+#include<bits/stdc++.h>
+using namespace std;
+#define N 9
+
+int grid[N][N];
+
+bool isSafeInRow(int& row,int value){
+    for(int i=0;i<N;i++){
+        if(grid[row][i]==value){
+            return false;
+        }
+    }
+    return true;
+}
+
+bool isSafeInCol(int& col,int value){
+    for(int i=0;i<N;i++){
+        if(grid[i][col]==value){
+            return false;
+        }
+    }
+    return true;
+}
+
+bool isSafeBox(int& row,int& col, int value){
+    int rowFactor=row-row%3;
+    int colFactor=col-col%3;
+    for(int i=0;i<3;i++){
+        for(int j=0;j<3;j++){
+            if(grid[i+rowFactor][j+colFactor]==value){
+                return false;
+            }
+        }
+    }
+    return true;
+}
+
+bool isSafe(int& row,int& col,int value){
+    if(isSafeInRow(row,value) && isSafeInCol(col,value) && isSafeBox(row,col,value)){
+        return true;
+    }
+    return false;
+}
+
+bool findEmptyLocation(int& x,int& y){
+    int Min=10,cnt=0;
+    for (int i = 0; i < N; i++)
+    {
+        for (int j = 0; j < N; j++)
+        {
+            if(grid[i][j]==0){
+                int cnt=0;
+                for(int k=1;k<=9;k++)
+                {
+                    if (isSafe(i,j,k))
+                        cnt++;
+                }
+                if(cnt<Min)
+                {
+                    Min=cnt;
+                    x=i;
+                    y=j;
+                }
+            }
+        }
+    }
+    if(Min<10)
+        return true;
+    return false;
+}
+
+
+
+bool solveSudoku(){
+    int row,col;
+    if(!findEmptyLocation(row,col)){
+        return true;
+    }
+    for(int i=1;i<=9;i++){
+        if(isSafe(row,col,i)){
+            grid[row][col]=i;
+            if(solveSudoku()){
+                return true;
+            }
+            grid[row][col]=0;
+        }
+    }
+    return false;
+}
+
+void solve(){
+    string inp[N];
+    string s;
+    getline(cin,s);
+    for(int i=0;i<N;i++){
+        getline(cin,inp[i]);
+    }
+    for(int i=0;i<N;i++){
+        for(int j=0;j<N;j++){
+            if(inp[i][j]=='.'){
+                grid[i][j]=0;
+            }
+            else grid[i][j]=inp[i][j]-'0';
+        }
+    }
+    solveSudoku();
+    for(int i=0;i<N;i++){
+        for(int j=0;j<N;j++){
+            cout<<grid[i][j];
+        }
+        cout<<endl;
+    }
+}
+
+
+
+int main(void){
+    int t;
+    cin>>t;
+    cin.ignore();
+    for(int i=1;i<=t;i++){
+        cout<<"Case "<<i<<": "<<endl;
+        solve();
+    }
+    return 0;
+}
+
+```