1314.Matrix_Block_Sum.cpp

/*
    written by Pankaj Kumar.
    country:-INDIA
    Institute: National Institute of Technology, Uttarakhand
*/
typedef long long ll ;
typedef unsigned long long ull;
typedef vector<ll> vl;
/*  Abbrevations  */
#define ff first
#define ss second
#define mp make_pair
#define pb push_back
#define Endl "\n"
#define all(V) (V).begin(),(V).end()
#define srt(V) sort(all(V))
#define srtGreat(V) sort(all(V),greater<ll>())
// some extra
#define printv(v) for(ll i=0;i<ll(v.size());i++){cout<<v[i]<<" ";} line;
#define precision(x) cout<<fixed<<setprecision(x);
#define sz(V) ll(V.size())

/* ascii value 
A=65,Z=90,a=97,z=122
*/
/*  --------------------MAIN PROGRAM----------------------------*/
// to run ctrl+b
const ll INF=LONG_MAX;
const ll mod1=1e9+7;
const ll mod2=998244353;

// Techniques
// divide into cases, brute force, pattern finding
// sort, greedy, binary search, two pointer
// transform into graph

// add main code here
class Solution {
public:
    vector<vector<int>> matrixBlockSum(vector<vector<int>>& mat, int k) {
        int n=mat.size();
        int m=mat[0].size();
        vector<vector<int>> dp(n+1,vector<int>(m+1,0));
        vector<vector<int>> result(n,vector<int>(m));
        int sum=0;
        for(int i=1;i<=n;i++){
            for(int j=1;j<=m;j++){
                dp[i][j]=dp[i][j-1]+dp[i-1][j]+mat[i-1][j-1]-dp[i-1][j-1];
            }
        }
        for(int i=1;i<=n;i++){
            for(int j=1;j<=m;j++){
                int start_r=max(1,i-k);
                int start_c=max(1,j-k);
                int end_r=min(i+k,n);
                int end_c=min(j+k,m);
                result[i-1][j-1]=dp[end_r][end_c]+dp[start_r-1][start_c-1]-(dp[end_r][start_c-1]+dp[start_r-1][end_c]);
            }
        }
        return result;
    }
};


/* -----------------END OF PROGRAM --------------------*/
/*
* stuff you should look before submission 
* constraint and time limit
* int overflow
* special test case (n=0||n=1||n=2)
* don't get stuck on one approach if you get wrong answer
*/