明明使用了pop(),但是并不能删除头元素，大小不发生变化？为什么呢？

Question

在主函数写个小测试是没问题的呀。下面是源代码：#include<iostream>using namespace std;#include<queue>class Graph{public: Graph(int v)//创建一个包含v个顶点但不包含边的图 { this->adjacent = new queue<int>[v]; this->V = v; this->E = 0; } int Vnum()//获取顶点的数量 { return this->V; } int Enum()//获取边的数量 { return this->E; } void addEdge(int v, int w)//向图中增加一条边 v-w { this->adjacent[v].push(w); this->adjacent[w].push(v); this->E++; } queue<int> adj(int v)//获取和顶点v相邻的所有顶点 { return this->adjacent[v]; }private: int V;//顶点数量 int E;//顶点边数量 queue<int> *adjacent;};class DepthFirstSearch{public: DepthFirstSearch(Graph G, int s) {//构件深度优先搜索对象，利用深度优先搜索找出G图中s顶点的所有相同顶点 this->marked = new bool[G.Vnum()]; for (int i = 0; i < G.Vnum(); ++i) { marked[i] = false; } this->N = 0; dfs(G, s); } void dfs(Graph G, int v)//利用深度优先搜索找出G中v顶点的所有相通顶点 { marked[v] = true; int w = G.adj(v).front(); while(!G.adj(v).empty())//找到v队列里的内容 { if (!marked[w]) { dfs(G, w); } cout << "队列大小："<<G.adj(v).size() << endl; G.adj(v).pop(); cout << "队列删除后的大小："<<G.adj(v).size() << endl; if (G.adj(v).empty() == 1) { break; } w = G.adj(v).front(); } this ->N++;//N加1 的位置放在当前节点变true的时候 } bool mark(int w)//判断w与s是否相通 { return marked[w]; } int count() { return N; }private: bool* marked;//索引代表顶点，值表示当前顶点是否已经被搜索 int N;//记录有多少个顶点与s顶点相同};void main(){ Graph g(13); g.addEdge(0, 6); g.addEdge(0, 2); g.addEdge(0, 1); g.addEdge(0, 6); g.addEdge(5, 3); g.addEdge(5, 4); g.addEdge(3, 4); g.addEdge(4, 6); g.addEdge(7, 8); g.addEdge(9, 10); g.addEdge(9, 12); g.addEdge(11, 12); g.addEdge(9, 11); DepthFirstSearch *DFS=new DepthFirstSearch(g, 0); int num= DFS->count(); cout << num << endl; /*queue<int> q; for (int i = 0; i < 5; i++) { q.push(i + 1); } cout << q.size() << endl; cout << q.empty() << endl; for (int i = 0; !q.empty(); i++) { q.pop(); } cout << q.size() << endl; cout << q.empty() << endl;*/ system("pause"); return;}

Answer 1

问题在于你的queue<int> adj(int v) 函数返回的是一个queue的拷贝，而不是queue本身。
改成
queue <int>& adj(int v) //获取和顶点v相邻的所有顶点
{
return adjacent[v];
}
全部源码如下：
#include<iostream>
#include<queue>
using namespace std;

class Graph {
public:
Graph(int v) //创建一个包含v个顶点但不包含边的图
{
this -> adjacent = new queue < int > [v];
this -> V = v;
this -> E = 0;
}
int Vnum() //获取顶点的数量 
{
return this -> V;
}
int Enum() //获取边的数量 
{
return this -> E;
}
void addEdge(int v, int w)
//向图中增加一条边 v-w 
{
this -> adjacent[v].push(w);
this -> adjacent[w].push(v);
this -> E++;
}
queue <int>& adj(int v) //获取和顶点v相邻的所有顶点
{
return adjacent[v];
}
private:
int V; //顶点数量 
int E; //顶点边数量 
queue < int > * adjacent;
};
class DepthFirstSearch {
public: 
DepthFirstSearch(Graph G, int s) { //构件深度优先搜索对象，利用深度优先搜索找出G图中s顶点的所有相同顶点 
this -> marked = new bool[G.Vnum()];
for (int i = 0; i < G.Vnum();
++i) {
marked[i] = false;
}
this -> N = 0;
dfs(G, s);
}
void dfs(Graph G, int v) //利用深度优先搜索找出G中v顶点的所有相通顶点 
{
marked[v] = true;
int w = G.adj(v).front();
while (!G.adj(v).empty()) //找到v队列里的内容 
{
if (!marked[w]) {
dfs(G, w);
}
cout << "队列大小：" << G.adj(v).size() << endl;
G.adj(v).pop();
cout << "队列删除后的大小：" << G.adj(v).size() << endl;
if (G.adj(v).empty() == 1) {
break;
}
w = G.adj(v).front();
}
this -> N++;
//N加1 的位置放在当前节点变true的时候 
}
bool mark(int w) //判断w与s是否相通 
{
return marked[w];
}
int count() {
return N;
}
private: bool * marked; //索引代表顶点，值表示当前顶点是否已经被搜索 
int N; //记录有多少个顶点与s顶点相同
};
int main() {
Graph g(13);
g.addEdge(0, 6);
g.addEdge(0, 2);
g.addEdge(0, 1);
g.addEdge(0, 6);
g.addEdge(5, 3);
g.addEdge(5, 4);
g.addEdge(3, 4);
g.addEdge(4, 6);
g.addEdge(7, 8);
g.addEdge(9, 10);
g.addEdge(9, 12);
g.addEdge(11, 12);
g.addEdge(9, 11);
DepthFirstSearch * DFS = new DepthFirstSearch(g, 0);
int num = DFS -> count();
cout << num << endl;
return 0;
}