leetcode/Problems/1314.cpp at master · DimitrijeDobrota/leetcode · GitHub

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class Solution {
    inline int clamp(const int a, const int x, const int y) { return min(max(a, x), y); }

  public:
    vector<vector<int>> matrixBlockSum(vector<vector<int>> &mat, int k) {
        int n = mat.size(), m = mat[0].size();
        vector<vector<int>> dp(n, vector<int>(m, 0)), res(n, vector<int>(m, 0));

        for (int i = 0, sum = 0; i < n; i++)
            sum = dp[i][0] = mat[i][0] + sum;
        for (int i = 0, sum = 0; i < m; i++)
            sum = dp[0][i] = mat[0][i] + sum;
        for (int i = 1; i < n; i++) {
            for (int j = 1; j < m; j++) {
                dp[i][j] = mat[i][j] + dp[i - 1][j] + dp[i][j - 1] - dp[i - 1][j - 1];
            }
        }

        for (int i = 0; i < n; i++) {
            for (int j = 0; j < m; j++) {
                int x1 = clamp(i - k, 0, n - 1), y1 = clamp(j - k, 0, m - 1);
                int x2 = clamp(i + k, 0, n - 1), y2 = clamp(j + k, 0, m - 1);
                res[i][j] = dp[x2][y2];
                if (x1 > 0) res[i][j] -= dp[x1 - 1][y2];
                if (y1 > 0) res[i][j] -= dp[x2][y1 - 1];
                if (x1 > 0 && y1 > 0) res[i][j] += dp[x1 - 1][y1 - 1];
            }
        }

        return res;
    }
};