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I used a recursive method, and I checked the case that X1 > X2, Y1>Y2 (the description says that (x1, Y1) is the upper-left and (X2, Y2) the lower right corner, though),


Just because it says (x1, y1) is the upper-left and (x2, y2) is the lower-right does not mean that x1 < x2, and y1 < y2. The ...