Codeforces Round #285 (Div. 1) A. Misha and Forest

从叶子节点开始广搜。。。。

#include <iostream>
#include <queue>
#include <stack>
#include <map>
#include <set>
#include <bitset>
#include <cstdio>
#include <algorithm>
#include <cstring>
#include <climits>
#include <cstdlib>
#include <cmath>
#include <time.h>
#define maxn 100005
#define maxm 200005
#define eps 1e-7
#define mod 1000000007
#define INF 0x3f3f3f3f
#define PI (acos(-1.0))
#define lowbit(x) (x&(-x))
#define mp make_pair
#define ls o<<1
#define rs o<<1 | 1
#define lson o<<1, L, mid 
#define rson o<<1 | 1, mid+1, R
#define pii pair<int, int>
#pragma comment(linker, "/STACK:16777216")
typedef long long LL;
typedef unsigned long long ULL;
using namespace std;
LL qpow(LL a, LL b){LL res=1,base=a;while(b){if(b%2)res=res*base;base=base*base;b/=2;}return res;}
LL powmod(LL a, LL b){LL res=1,base=a;while(b){if(b%2)res=res*base%mod;base=base*base%mod;b/=2;}return res;}
//head

int xorsum[maxn];
int du[maxn];
queue<int> q;
vector<pair<int, int> > ans;
int n;

void read()
{
	scanf("%d",  &n);
	for(int i = 0; i < n; i++) scanf("%d%d", &du[i], &xorsum[i]);
}

void work()
{
	for(int i = 0; i < n; i++) if(du[i] == 1) q.push(i);
	while(!q.empty()) {
		int u = q.front();
		q.pop();
		if(du[u] == 0) continue;
		int v = xorsum[u];
		xorsum[v] ^= u;
		ans.push_back(mp(u, v));
		du[v]--;
		if(du[v] == 1) q.push(v);
	}
	printf("%d\n", (int)ans.size());
	for(int i = 0; i < ans.size(); i++) printf("%d %d\n", ans[i].first, ans[i].second);
}

int main()
{
	read();
	work();
	
	return 0;
}