【洛谷luogu】P5018-对称二叉树（递归）_洛谷p5018

原题链接：https://www.luogu.org/problemnew/show/P5018

题目大意

给定二叉树，求其最大的对称子树。

解法

与题解中流传的解法类似，主要是复杂度不好估计。

#include <iostream>
#include <ctime>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <vector>
#include <queue>

#define forn(i, n) for (int i = 0; i < (int)(n); ++i)
#define for1(i, n) for (int i = 1; i <= (int)(n); ++i)
#define pb push_back

using namespace std;
typedef long long LL;
const int N=1000000+5;

struct Node{
	int w;
	int l,r;
}Tree[N];

bool issame(int x,int y){
	if(x==-1&&y==-1) return 1;
	if(x==-1||y==-1) return 0;
	if(Tree[x].w!=Tree[y].w) return 0;
	if(!issame(Tree[x].l,Tree[y].r)) return 0;
	if(!issame(Tree[x].r,Tree[y].l)) return 0;
	return 1;
}
int d[N],ans=0,n;
void dfs(int u){
	int l=Tree[u].l,r=Tree[u].r;
	if(l==-1&&r==-1){
		d[u]=1;
		return;
	}
	if(l!=-1) dfs(l);
	if(r!=-1) dfs(r);
	d[u]=(l!=-1?d[l]:0)+(r!=-1?d[r]:0)+1;
//	cout<<u<<" "<<d[u]<<endl;
}

void dfs_(int u){
	int l=Tree[u].l,r=Tree[u].r;
	if(l==-1&&r==-1){
		ans=max(ans,1);
		return;
	}
	if(issame(l,r)){
		ans=max(ans,1+d[l]+d[r]);
		return ;
	}
	if(l!=-1) dfs_(l);
	if(r!=-1) dfs_(r);
}

int main() {

#ifdef LOCAL_DEFINE
	freopen("P5018.in", "rt", stdin);
#endif
	scanf("%d",&n);
	for1(i,n) scanf("%d",&Tree[i].w);
	for1(i,n) scanf("%d%d",&Tree[i].l,&Tree[i].r);
	dfs(1);
	dfs_(1);
	printf("%d\n",ans);

#ifdef LOCAL_DEFINE
    cerr << "Time elapsed: " << 1.0 * clock() / CLOCKS_PER_SEC << " s.\n";
#endif
    return 0;
}
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73