Shouldn’t we first check whether i-1 & j-1 exist and then add and subtract their related elements(pre[i-1][j-1] pre[i-1][y] pre[x][j-1]) to the sum because they could be out of bound.
Related to accessing index i-1 and j-1 in sum of all sub matrices using pre sum matrix
yes we need to check the cases (i) i and j both = 0 (ii) if i==0 only (iii) if j==0 only and accordingly write our code.
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