LeetCode 886 Possible Bipartition - Building the Graph

The input gives you an edge list: dislikes = [[1,2],[1,3],[2,4]] means person $1$ dislikes $2$, person $1$ dislikes $3$, person $2$ dislikes $4$. You need an adjacency list to run BFS. Create graph as an array of $n+1$ empty lists (people are $1$-indexed).

For each pair [a, b], add $b$ to graph[a] and $a$ to graph[b]. Dislikes are mutual, so edges go both ways. This converts the input into a graph you can traverse.