rt。

WA on Test 7

但好像都没有警钟里的问题。

#include <bits/stdc++.h>
using namespace std;

#define int long long
#define endl '\n'
#define debug(x) cerr << #x << " = " << x << endl
#define rep(i, a, b) for (int i = (a); i <= (b); i++)
#define per(i, a, b) for (int i = (a); i >= (b); i--)
#define gn(u, v) for (int v : G.G[u])
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define sz(x) (int)(x).size()
#define pii pair<int, int>
#define vi vector<int>
#define vpii vector<pii>
#define vvi vector<vi>
#define no cout << "NO" << endl
#define yes cout << "YES" << endl
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define tomin(x, y) ((x) = min((x), (y)))
#define tomax(x, y) ((x) = max((x), (y)))
#define ck(mask, i) (((mask) >> (i)) & 1)
#define pq priority_queue

constexpr int MAXN = 1e5 + 5;

int n, m;
string a;

vector<int> operator+(const vector<int>& a, const vector<int>& b) {
    vector<int> res(26, 0);
    rep(i, 0, 25) {
        res[i] = a[i] + b[i];
    }
    return res;
}

struct SegTree {
    struct Node {
        int l, r, lz;
        vector<int> cnt;
    } tr[MAXN * 4];

    void pushup(int k) {
        rep(i, 0, 25) {
            tr[k].cnt[i] = tr[k * 2].cnt[i] + tr[k * 2 + 1].cnt[i];
        }
    }

    void lazy(int k, int v) {
        // assert(!v);
        tr[k].lz = v;
        rep(i, 0, 25) {
            if (v == i) {
                tr[k].cnt[i] = (tr[k].r - tr[k].l + 1);
            } else {
                tr[k].cnt[i] = 0;
            }
        }
    }

    void pushdown(int k) {
        if (!tr[k].lz)
            return;
        lazy(k * 2, tr[k].lz);
        lazy(k * 2 + 1, tr[k].lz);
        tr[k].lz = 0;
    }

    void build(int k, int l, int r) {
        tr[k] = { l, r, 0, vector<int>(26, 0) };
        tr[k].cnt[a[l] - 'a']++;
        if (l == r)
            return;
        int mid = (l + r) >> 1;
        build(k * 2, l, mid);
        build(k * 2 + 1, mid + 1, r);
        pushup(k);
    }

    void update(int k, int l, int r, int v) {
        // cerr << k << " " << l << " " << r << " " << v << " " << tr[k].l << " " << tr[k].r << endl;
        if (tr[k].l >= l && tr[k].r <= r)
            return lazy(k, v);
        pushdown(k);
        int mid = (tr[k].l + tr[k].r) >> 1;
        int lc = k * 2, rc = k * 2 + 1;
        if (r <= mid)
            update(lc, l, r, v);
        else if (l > mid)
            update(rc, l, r, v);
        else
            update(lc, l, mid, v), update(rc, mid + 1, r, v);
        pushup(k);
    }

    vector<int> query(int k, int l, int r) {
        if (tr[k].l >= l && tr[k].r <= r)
            return tr[k].cnt;
        pushdown(k);
        int mid = (tr[k].l + tr[k].r) >> 1;
        int lc = k * 2, rc = k * 2 + 1;
        if (r <= mid)
            return query(lc, l, r);
        else if (l > mid)
            return query(rc, l, r);
        else
            return query(lc, l, mid) + query(rc, mid + 1, r);
    }
} seg;

void color(int l, int r, map<int, int>& col) {
    int sum = 0;
    for (auto [k, v] : col) {
        if (v == 0) continue;
        sum += v;
        seg.update(1, l + sum - v, l + sum - 1, k);
        seg.update(1, r - sum + 1, r - sum + v, k);
    }
}

signed main() {
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);

    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);

    cin >> n >> m;
    cin >> a;
    a = " " + a;

    seg.build(1, 1, n);

    rep(i, 0, m - 1) {
        int l, r;
        cin >> l >> r;
        auto cnt = seg.query(1, l, r);
        int odd = 26;
        rep(j, 0, 25) {
            if (cnt[j] % 2 == 1) {
                if (odd != 26) {
                    odd = 27;
                    break;
                }
                odd = j;
            }
        }
        if (odd == 27) continue;
        map<int, int> col;
        if (odd == 26) {
            rep(j, 0, 25) col[j] = cnt[j] / 2;
        } else {
            rep(j, 0, 25) {
                if (j != odd) {
                    col[j] = cnt[j] / 2;
                }
            }
            int mid = l + r >> 1;
            seg.update(1, mid - cnt[odd] / 2, mid + cnt[odd] / 2, odd);
        }
        color(l, r, col);
    }

    rep(i, 1, n) {
        auto cnt = seg.query(1, i, i);
        for (int j = 0; j < 26; j++) {
            if (cnt[j]) cout << (char)(j + 'a');
        }
    }
    cout << endl;

    return 0;
}