pat1127 ZigZagging on a Tree

题意：输入一颗树的中序遍历和后序遍历，按层按照一层从左到右一层从右到左交替输出。

思路：建树，得到层序，每个点记录一下层次，遍历层序交替倒置序列即可。

代码

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

using namespace std;

const int MAX_N = 35;
int postorder[MAX_N], inorder[MAX_N];
int lc[MAX_N], rc[MAX_N], level[MAX_N];
int n, root;
int ans[MAX_N], len;

int build(int pl, int pr, int il, int ir) {
    if (pl >= pr) return -1;
    if (pr - pl == 1) return pl;
    int p = -1;
    for (int i = il; i < ir; i++) {
        if (inorder[i] == postorder[pr-1]) {
            p = i; break;
        }
    }
    lc[pr-1] = build(pl, pl+(p-il), il, p);
    rc[pr-1] = build(pl+(p-il), pr-1, p+1, ir);
    return pr-1;
}

void pre() {
    queue<int> q; len = 0;
    q.push(root); level[root] = 0;
    while (!q.empty()) {
        int t = q.front(); q.pop();
        if (lc[t] != -1) { level[lc[t]] = level[t]+1; q.push(lc[t]); }
        if (rc[t] != -1) { level[rc[t]] = level[t]+1; q.push(rc[t]); }
        ans[len++] = t;
    }
    int lo = 1, le = 1, flag = 0;
    for (int i = 1; i < len; i++) {
        if (level[ans[i]] != le) {
            if (flag) reverse(ans+lo, ans+i);
            flag = flag ^ 1;
            lo = i;
            le = level[ans[i]];
        }
    }
    if (flag) reverse(ans+lo, ans+len);
}

int main() {
    // freopen("in.txt", "r", stdin);
    // freopen("out.txt", "w", stdout);
    scanf("%d", &n);
    memset(lc, -1, sizeof(lc)); memset(rc, -1, sizeof(rc));
    for (int i = 0; i < n; i++) scanf("%d", &inorder[i]);
    for (int i = 0; i < n; i++) scanf("%d", &postorder[i]);
    root = build(0, n, 0, n);
    pre();
    for (int i = 0; i < len; i++) {
        if (i > 0) printf(" ");
        printf("%d", postorder[ans[i]]);
    }
    printf("\n");
    return 0;
}