We know the prefix array and S [i] = S [i-1] + a [i]
So, how to find it and the two-dimensional prefix?
And two-dimensional prefix:
Prefix and is yellow, red, gray and green right point and green point
How to calculate?
s[i][j]=s[i-1][j]+s[i][j-1]-s[i-1][j-1]+a[i][j];
Prefix portion and green = (red + gray) + (gray yellow +) - Gray Green +
With prefix and, how to calculate the point and the right to a certain part of it?
Similarly, the sum of the green parts now = S (4,4) -S (4,2) -S (2,4) + S (2,2)
We set a side portion of the green rx * ry, a sum, the green part of the lower right corner coordinates i, j
a[i][j]=s[i][j]-s[i-rx-1][j]-s[i][j-1-ry]+s[i-1-rx][j-1-ry];