func criticalConnections(n int, connections [][]int) [][]int {
    graph := make([][]int, n)
    time := make([]int, n)
    parTime := make([]int, n)
    par := make([]int, n)
    ret := make([][]int, 0)

    for i := range graph {
        graph[i] = make([]int, 0)
        time[i] = -1
        parTime[i] = -1
    }

    for i := range connections {
        graph[connections[i][0]] = append(graph[connections[i][0]], connections[i][1])
        graph[connections[i][1]] = append(graph[connections[i][1]], connections[i][0])
    }

    dfs(graph, time, parTime, par, 0, &ret)

    return ret
}

var t int = 0

func dfs(graph [][]int, time[]int, parTime[]int, par[]int, src int, ret *[][]int) {
    t++
    time[src] = t
    parTime[src] = t

    for i := range graph[src] {
        if time[graph[src][i]] == -1 {
            par[graph[src][i]] = src
            dfs(graph, time, parTime, par, graph[src][i], ret)
            if parTime[graph[src][i]] > time[src] {
                *ret = append(*ret, []int{src, graph[src][i]})
            } else {
                if parTime[src] > parTime[graph[src][i]] {
                    parTime[src] = parTime[graph[src][i]]
                }
            }
        } else if par[src] != graph[src][i] {
            if parTime[src] > time[graph[src][i]] {
                parTime[src] = time[graph[src][i]]
            }
        }
    }
}