flood-fill

char g[256][256];
int sx[]={1,-1,0,0},sy[]={0,0,1,-1};
void rec(int x,int y,char C,char H){
    if(x<0||x>N||y<0||y>M||g[x][y]!=H)return;
    g[x][y]=C;
    for(int i(0);i<4;++i)rec(x+sx[i],y+sy[i],C,H);
}