View Compilation Error

0_0_15969811_2529.cpp: In function 'void Add_Edge(int, int)':
0_0_15969811_2529.cpp:19:2: error: '_Next' was not declared in this scope
  _Next[idx] = head[f]; to[idx] = t; head[f] = idx++;
  ^
0_0_15969811_2529.cpp: In function 'void dfs1(int, int, int)':
0_0_15969811_2529.cpp:28:31: error: '_Next' was not declared in this scope
  for(int i = head[u]; ~i; i = _Next[i])
                               ^
0_0_15969811_2529.cpp: In function 'void dfs2(int, int)':
0_0_15969811_2529.cpp:46:31: error: '_Next' was not declared in this scope
  for(int i = head[u]; ~i; i = _Next[i])
                               ^