Visualizing the Trace

Graph: $1−2$, $1−3$, $2−4$. Start dfs($1$):

$1.$ Mark $1$. List: {$2$, $3$}.

$2.$ Go to $2$.

Call dfs($2$). $2.1$ Mark $2$. List: {$1$, $4$}. $2.2$ Neighbor $1$ is visited. Skip. $2.3$ Neighbor $4$ is not. Call dfs($4$). $2.3.1$ Mark $4$. List: {$2$}. $2.3.2$ Neighbor $2$ is visited. Skip. $2.3.3$ Done. Return to dfs($2$). $2.4$ dfs($2$) Done. Return to dfs($1$).

$3.$ Back at $1$.

Next neighbor is $3$. Call dfs($3$). Path: $1→2→4…back⋯→3$.