pat1143 Lowest Common Ancestor

题意：给出一个bst的先序，给一些点对，求这些点对的lca。

思路：建树，lca必然在两个点对值中间，根据这个性质搜索树。

代码

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <cmath>

using namespace std;

const int MAX_N = 10100;
int M, N, x, y, r;
int a[MAX_N];
int lc[MAX_N], rc[MAX_N];
bool f1, f2;

int build(int lo, int hi) {
    if (lo >= hi) return -1;
    if (hi-lo==1) return lo;
    int p = hi;
    for (int i = lo+1; i < hi; i++) {
        if (a[lo] <= a[i]) {
            p = i; break;
        }
    }
    lc[lo] = build(lo+1, p);
    rc[lo] = build(p, hi);
    return lo;
}

bool Find(int r, int x) {
    if (r == -1) return false;
    if (a[r] == x) return true;
    if (a[r] < x) return Find(rc[r], x);
    else return Find(lc[r], x);
}

int lca(int x, int y, int r) {
    if (y < x) swap(x, y);
    while (!(x<=a[r]&&a[r]<=y)) {
        if (a[r] < x) {
            r = rc[r];
        } else if (a[r] > y) {
            r = lc[r];
        }
    }
    return r;
}

int main() {
    // freopen("in.txt", "r", stdin);
    // freopen("out.txt", "w", stdout);
    scanf("%d %d", &M, &N);
    memset(lc, -1, sizeof(lc)); memset(rc, -1, sizeof(rc));
    for (int i = 0; i < N; i++) {
        scanf("%d", &a[i]);
    }
    r = build(0, N);
    for (int i = 0; i < M; i++) {
        scanf("%d %d", &x, &y);
        f1 = Find(r, x); f2 = Find(r, y);
        if (!f1 || !f2) {
            if (!f1 && !f2) {
                printf("ERROR: %d and %d are not found.\n", x, y);
            } else if (!f1) {
                printf("ERROR: %d is not found.\n", x);
            } else {
                printf("ERROR: %d is not found.\n", y);
            }
        } else {
            int t = lca(x, y, r);
            if (a[t] == x) {
                printf("%d is an ancestor of %d.\n", x, y);
            } else if (a[t] == y) {
                printf("%d is an ancestor of %d.\n", y, x);
            } else {
                printf("LCA of %d and %d is %d.\n", x, y, a[t]);
            }
        }
    }
    return 0;
}