Tarjan Example

We trace Tarjan on graph: $1 \to 2 \to 3 \to 1$, $3 \to 4$. Start DFS from $1$: disc[1] = 0, low[1] = 0, push $1$ to stack. Visit $2$: disc[2] = 1, low[2] = 1, push $2$. Visit $3$: disc[3] = 2, low[3] = 2, push $3$. Back edge $3 \to 1$: update low[3] = 0, low[2] = 0, low[1] = 0.

Visit $4$: disc[4] = 3, low[4] = 3, push $4$. Since low[4] = disc[4], pop $\{4\}$ as one SCC. Backtrack to $1$: low[1] = disc[1], pop $\{1, 2, 3\}$ as another SCC.