Matrix Block Sum LeetCode solution in JavaScript

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/**
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 * @param {number[][]} mat
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 * @param {number} k
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 * @return {number[][]}
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 */
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var matrixBlockSum = function (mat, k) {
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  let sum = 0;
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  let dp = Array(mat.length + 1);
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  dp[0] = Array(mat[0].length).fill(0);
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  // sum of row el
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  for (let i = 0; i < mat.length; i++) {
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    dp[i + 1] = Array(mat[0].length).fill(0);
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    for (let j = 0; j < mat[0].length; j++) {
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      dp[i + 1][j] += mat[i][j] + sum;
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      sum = dp[i + 1][j];
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    }
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    sum = 0;
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  }
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  // sum of col el
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  for (let j = 0; j < mat[0].length; j++) {
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    for (let i = 0; i < mat.length; i++) {
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      dp[i + 1][j] += sum;
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      sum = dp[i + 1][j];
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    }
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    sum = 0;
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  }
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  dp = dp.slice(1);
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  // cal sum of blocks
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  for (let i = 0; i < mat.length; i++) {
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    let r1 = Math.max(0, i - k);
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    let r2 = Math.min(mat.length - 1, i + k);
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    for (let j = 0; j < mat[0].length; j++) {
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      let c1 = Math.max(0, j - k);
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      let c2 = Math.min(mat[0].length - 1, j + k);
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      let value = dp[r2][c2];
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      if (r1 - 1 >= 0) {
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        value -= dp[r1 - 1][c2];
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      }
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      if (c1 - 1 >= 0) {
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        value -= dp[r2][c1 - 1];
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      }
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      if (r1 - 1 >= 0 && c1 - 1 >= 0) {
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        value += dp[r1 - 1][c1 - 1];
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      }
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      mat[i][j] = value;
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    }
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  }
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  return mat;
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};