Language: C++

//header
#ifdef LOCAL
    #include "cxx-prettyprint-master/prettyprint.hpp"
    #define print(x) cout << x << endl
#else
    #define print(...) 42
#endif
    #pragma GCC optimize("Ofast")
    #include <bits/stdc++.h>
    //types
    using namespace std;
    using ll = long long;
    typedef pair < ll , ll >Pl;
    typedef pair < int, int >Pi;
    typedef vector<ll> vl;
    typedef vector<int> vi;
    template< typename T >
    using mat = vector< vector< T > >;
    template <std::uint_fast64_t Modulus> class modint {
        using u64 = std::uint_fast64_t;

        public:
        u64 a;

        constexpr modint(const u64 x = 0) noexcept : a(x % Modulus) {}
        constexpr u64 &value() noexcept { return a; }
        constexpr const u64 &value() const noexcept { return a; }
        constexpr modint operator+(const modint rhs) const noexcept {
            return modint(*this) += rhs;
        }
        constexpr modint operator-(const modint rhs) const noexcept {
            return modint(*this) -= rhs;
        }
        constexpr modint operator*(const modint rhs) const noexcept {
            return modint(*this) *= rhs;
        }
        constexpr modint operator/(const modint rhs) const noexcept {
            return modint(*this) /= rhs;
        }
        constexpr modint &operator+=(const modint rhs) noexcept {
            a += rhs.a;
            if (a >= Modulus) {
            a -= Modulus;
            }
            return *this;
        }
        constexpr modint &operator-=(const modint rhs) noexcept {
            if (a < rhs.a) {
            a += Modulus;
            }
            a -= rhs.a;
            return *this;
        }
        constexpr modint &operator*=(const modint rhs) noexcept {
            a = a * rhs.a % Modulus;
            return *this;
        }
        constexpr modint &operator/=(modint rhs) noexcept {
            u64 exp = Modulus - 2;
            while (exp) {
            if (exp % 2) {
                *this *= rhs;
            }
            rhs *= rhs;
            exp /= 2;
            }
            return *this;
        }
        };
    //abbreviations
    #define all(x) (x).begin(), (x).end()
    #define rall(x) (x).rbegin(), (x).rend()
    #define rep2(i,a,b) for(int i=(a);i<(b);++i)
    #define rep(i,n) for(int i=0;i<(n);++i)
    #define rev(i,n) for(int i=n-1;i>=0;i--)
    #define SZ(x) ((ll)(x).size())
    #define pb(x) push_back(x)
    #define ff first
    #define ss second
    #define mp make_pair
    //functions
    ll gcd(ll a, ll b) { return b ? gcd(b, a%b) : a; }
    template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
    template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }
    template< typename T >
    T mypow(T x, ll n) {
        T ret = 1;
        while(n > 0) {
            if(n & 1) (ret *= x);
            (x *= x);
            n >>= 1;
        }
        return ret;
    }
    uint64_t my_rand(void) {
        static uint64_t x = 88172645463325252ULL;
        x = x ^ (x << 13); x = x ^ (x >> 7);
        return x = x ^ (x << 17);
    }
    //graph template
    template< typename T >
    struct edge {
        int src, to;
        T cost;

        edge(int to, T cost) : src(-1), to(to), cost(cost) {}

        edge(int src, int to, T cost) : src(src), to(to), cost(cost) {}

        edge &operator=(const int &x) {
        to = x;
        return *this;
    }

    operator int() const { return to; }
    };

    template< typename T >
    using Edges = vector< edge< T > >;
    template< typename T >
    using WeightedGraph = vector< Edges< T > >;
    using UnWeightedGraph = vector< vector< int > >;

//constant
#define INF 1'010'000'000'000'000'017LL
#define mod 1000000007LL
typedef modint<mod> mint;
#define eps 0.000001
const double PI = M_PI;
//library

//main
int main(){
    int n; cin>>n;
    mat<int> g(n);
    vi cnt(n);
    rep(i, n-1){
        int a, b; cin>>a>>b;
        g[a-1].pb(b-1);
        g[b-1].pb(a-1);
        cnt[a-1]++;cnt[b-1]++;
    }
    vi par(n, -1);
    vi que = {0};
    par[0] = 1;
    while(que.size()){
        int v = que.back();
        int p = par[v];
        que.pop_back();
        for(auto nv:g[v]){
            if(par[nv]>=0)continue;
            par[nv] = 3-p;
            que.pb(nv);
        }
    }
    int c = 0;
    mat<int> nums(3);
    rep(i, n){
        if(par[i] == 1){
            c++;
        }
        nums[(i+1)%3].pb(i+1);
    }
    int r = n-c*2;
    int p = min(abs(r), n/3);
    c = 2;
    if(r<=0){
        c = 1;
    }
    vi ans(n);
    rep(i, n){
        if(cnt[i] == 1&&p>1&&par[i] == c){
            if(ns[g[i][0]]>0)continue;
            par[i] = 3-c;
            ans[g[i][0]] = nums[0].back();
            nums[0].pop_back();
            p-=2;
        }
    }
    rep(i, n){
        if(ans[i]>0){
            cout << ans[i] << ' ';
            continue;
        }
        if(nums[par[i]].size() == 0)par[i] = 0;
        ans[i] = nums[par[i]].back();
        nums[par[i]].pop_back();
        cout << ans[i] << ' ';
    }
    cout << endl;
}   


Compiler output:
a.cc: In function 'int main()':
a.cc:173:16: error: 'ns' was not declared in this scope; did you mean 'ans'?
  173 |             if(ns[g[i][0]]>0)continue;
      |                ^~
      |                ans