Sum of Distances in Tree

export function sumOfDistancesInTree(n: number, edges: number[][]): number[] {
  const tree: number[][] = new Array(n).fill(undefined).map(() => []);
  for (const [a, b] of edges) {
    tree[a].push(b);
    tree[b].push(a);
  }
 
  const answer = new Array<number>(n).fill(0);
  const count = new Array<number>(n).fill(1);
 
  const dfs = (node: number, parentNode?: number) => {
    for (const childNode of tree[node]) {
      if (childNode !== parentNode) {
        dfs(childNode, node);
        count[node] += count[childNode];
        answer[node] += answer[childNode] + count[childNode];
      }
    }
  };
 
  const dfs2 = (node: number, parentNode?: number) => {
    for (const childNode of tree[node]) {
      if (childNode !== parentNode) {
        answer[childNode] = answer[node] - count[childNode] + n - count[childNode];
        dfs2(childNode, node);
      }
    }
  };
 
  dfs(0);
  dfs2(0);
 
  return answer;
}