CSES 1133 Tree Distances II - Implementation

Here is the solution:

subtreeSum = array of size n + 1
subtreeSize = array of size n + 1
ans = array of size n + 1

function dfsDown(v, parent):
    subtreeSize[v] = 1
    subtreeSum[v] = 0
    for u in adj[v]:
        if u != parent:
            dfsDown(u, v)
            subtreeSize[v] = subtreeSize[v] + subtreeSize[u]
            subtreeSum[v] = subtreeSum[v] + subtreeSum[u] + subtreeSize[u]

function dfsUp(v, parent, total):
    ans[v] = total
    for u in adj[v]:
        if u != parent:
            dfsUp(u, v, total - subtreeSize[u] + (n - subtreeSize[u]))

dfsDown(1, 0)
dfsUp(1, 0, subtreeSum[1])

This runs in $O(n)$ time and uses $O(n)$ space.