Longest Cycle - Dynamic Programming / Tree

Problems
1

https://algo.monster/problems/longest_cycle

2

JavaScript solution:

const dfs = (graph, current, parent, longestPathTotal, longestPathFromLeafNode) => {
    longestPathFromLeafNode[current] = 0;
    longestPathTotal[current] = 0;
    let longestPath = 0;
    let secondLongestPath = 0;
    for (const neighbor of graph[current]) {
        if (neighbor != parent) {
            longestPathTotal[current] = Math.max(longestPathTotal[current], dfs(graph, neighbor, current, longestPathTotal, longestPathFromLeafNode));
            longestPathFromLeafNode[current] = Math.max(longestPathFromLeafNode[current], longestPathFromLeafNode[neighbor] + 1);
            if (longestPathFromLeafNode[current] > longestPath) {
                secondLongestPath = longestPath;
                longestPath = longestPathFromLeafNode[current];
            }
            else if (longestPathFromLeafNode[current] > secondLongestPath) {
                secondLongestPath = longestPathFromLeafNode[current];
            }
        }
    }
    longestPathTotal[current] = Math.max(longestPathTotal[current], longestPath + secondLongestPath);
    return longestPathTotal[current];
}
const longestCycle = (n, edges) => {
    const longestPathTotal = new Array(n + 1, 0)
    const longestPathFromLeafNode = new Array(n+1, 0)

    const graph = generateGraph(n, edges);
    return dfs(graph, 1, 1, longestPathTotal, longestPathFromLeafNode) + 1;
}
3

Hey, please why did we choose to construct an undirected graph over a directed graph?