https://atcoder.jp/contests/dp/tasks/dp_g

longestPath.js

const longestPath = (adjList) => {

    const graph = {};
    adjList.forEach(([from, to]) => {
        if(!graph[from]) graph[from] = new Set();
        graph[from].add(to);
    });

    const memo = {}; // node -> longestPathLength starting at node
    let result = 0;
    for(let start in graph){
        if(!memo.hasOwnProperty(start)) memo[start] = _longestPath(start);
        result = Math.max(result, memo[start]);
    }
    return result;

    function _longestPath(startNode){

        if(memo.hasOwnProperty(startNode)) return memo[startNode];
        if(!graph[startNode]) return 0;

        let maxPathLength = 0;
        for(let adj of graph[startNode]){
            maxPathLength = Math.max(maxPathLength, 1 + _longestPath(adj));
        }

        return memo[startNode] = maxPathLength;
    };
};