we trace Tarjan on graph: , . Start DFS from : disc[1] = 0, low[1] = 0, push to stack. Visit : disc[2] = 1, low[2] = 1, push . Visit : disc[3] = 2, low[3] = 2, push . Back edge : update low[3] = 0, low[2] = 0, low[1] = 0.
Visit : disc[4] = 3, low[4] = 3, push . Since low[4] = disc[4], pop as one SCC. Backtrack to : low[1] = disc[1], pop as another SCC.