笛卡尔树

http://www.hhanger.com/?p=134

http://www.coderplusplus.com/?p=393

POJ 2201

const int N = 50010;
int stack[N];
int pre[N], lson[N], rson[N];

struct node {
    int key;
    int val;
    int id;
} st[N];

bool operator < (const node& a, const node& b) {
    return a.key < b.key;
}

void init(int n) {
    memset(stack, -1, sizeof(stack));
    memset(pre, 0, sizeof(pre));

    for(int i = 0; i <= n; ++i) {
        lson[i] = rson[i] = 0;
    }
}

void build_cartesian_tree(int n) {
    int i, k, top = -1;
    for(i = 0; i < n; ++i) {
        k = top;
        while(k >= 0 && st[stack[k] + 1].val > st[i + 1].val)   --k;
        if(k != -1) {
            pre[st[i + 1].id] = st[stack[k] + 1].id;
            rson[st[stack[k] + 1].id] = st[i + 1].id;
        }
        if(k < top) {
            pre[st[stack[k + 1] + 1].id] = st[i + 1].id;
            lson[st[i + 1].id] = st[stack[k + 1] + 1].id;
        }
        stack[++k] = i;
        top = k;
    }
    pre[st[stack[0] + 1].id] = 0;
}

int main() {
    //freopen("data.in", "r", stdin);

    int n, i;
    while(~scanf("%d", &n)) {
        init(n);
        for(i = 1; i <= n; ++i) {
            scanf("%d%d", &st[i].key, &st[i].val);
            st[i].id = i;
        }
        sort(st + 1, st + n + 1);
        build_cartesian_tree(n);

        puts("YES");
        for(i = 1; i <= n; ++i) {
            printf("%d %d %d\n", pre[i], lson[i], rson[i]);
        }
    }
    return 0;
}