Trace image with start (1,1).
Original color at (1,1) is 1. New color is 2.
DFS from (1,1): color is 1, change to 2. Visit neighbors.
- (0,1): color 1, change to 2. Visit neighbors.
- (0,0): color 1, change to 2. Neighbors: (0,1) already 2, (1,0) is 1.
- (0,2): color 1, change to 2. Continue DFS from that cell.
- (1,0): color 1, change to 2...
- (2,1): color 0, skip (wrong color).
Result: all connected 1's changed to 2.
Each cell visited once. time. Recursion stack: worst case.