Find if there is a path between two vertices in a directed graph

基础遍历预热

#include <iostream>
#include <vector>
#include <stack>
#include <algorithm>
using namespace std;


struct TreeNode {
    int val;
    TreeNode* left;
    TreeNode* right;
    TreeNode(int x): val(x), left(nullptr), right(nullptr) {}
};


// 前序遍历 递归
class SolutionRecursive {
public:
    void traverse(TreeNode* cur, vector<int>& vec) {
        if (cur == nullptr) {
            return;
        }
        vec.push_back(cur->val);
        traverse(cur->right, vec);
        traverse(cur->left, vec);
    }

    vector<int> preOrderTraveral(TreeNode* root) {
        vector<int> result;
        traverse(root, result);
        return result;
    }

};


// 前序遍历 迭代
class SolutionIter {
    vector<int> preOrderTraversal(TreeNode* root) {
        stack<TreeNode*> st;
        vector<int> result;
        if (root == nullptr) {
            return result;
        }
        st.push(root);
        while( !st.empty()) {
            TreeNode* Node = st.top();
            st.pop();
            result.push_back(Node->val);  // 根
            if (Node->right) {
                st.push(Node->right);  // 左
            }
            if (Node->left) {
                st.push(Node->left);  // 右
            }
        }
        return result;
    }
};


// 后续遍历 迭代, 左、右、根
class Solution {
public:
    vector<int> PostOrderTraversal(TreeNode* root) {
        stack<TreeNode*> st;
        vector<int> result;
        if (root == nullptr) {
            return result;
        }
        st.push(root);
        while (!st.empty()) {
            TreeNode* Node = st.top();
            st.pop();
            result.push_back(Node->val);
            if (Node->left) {
                st.push(Node->left);
            }
            if (Node->right) {
                st.push(Node->right);
            }
        }
        reverse(result.begin(), result.end());
        return result;
    }
};

BFS热身（二叉树）

#include <queue>
#include <vector>

class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        if (!root) {
            return {};
        }
        queue<TreeNode*> que;
        if (root != nullptr) {
            que.push(root);
        }
        vector<vector<int>> result;
        while (!que.empty()) {
            int size = que.size();
            vector<int> vec;
            for (int i = 0; i < size; i++) {
                TreeNode* Node = que.front();
                que.pop();
                vec.push_back(Node->val);
                if (Node->left) {
                    que.push(Node->left);
                }
                if (Node -> right) {
                    que.push(Node->right);
                }
            }
            result.push_back(vec);
        }
        return result;
    }
};

正式上菜，有向图中2个节点

 

C++  BFS(图 遍历)

 path between two vertices in directed graph

#include <iostream>
#include <list>

using namespace std;

class Graph {
    int V;
    list<int> *adj;
public:
    Graph(int V);

    void addEdge(int v, int w); // function to add an edge to graph
    bool isReachable(int s, int d);

};

Graph::Graph(int V) {
    this->V = V;
    adj = new list<int>[V];
}

void Graph::addEdge(int v, int w) {
    adj[v].push_back(w);
}

// use bfs to write code
bool Graph::isReachable(int s, int d) {
    if (s == d)
        return true;

    // 存储节点状态 visited, list -> queue
    // 遍历 iterator
    // 某个节点bfs，判断是否出现另一个节点
    bool *visited = new bool[V];
    for (int i = 0; i < V; i++) {
        visited[i] = false;
    }
    list<int> queue;
    queue.push_back(s);

    list<int>::iterator i;

    while (!queue.empty()) {
        s = queue.front();
        queue.pop_front();

        for (i = adj[s].begin(); i != adj[s].end(); i++) {
            if (*i == d)
                return true;
            // 考虑是否visited
            if (!visited[*i]) {
                visited[*i] = true;
                queue.push_back(*i);
            }
        }
    }
    return false;
}

path true: 1 to 3

path false: 3 to 1

python

# 有向图中的2节点是否联通问题
from collections import defaultdict
class Graph:
    """
    这里比较粗暴的认为，0， 1， 2， 3， 默认为node节点，实际场景中有些不妥
    """
    def __init__(self, vertices):
        self.V = vertices
        self.graph = defaultdict(list)

    def addEdge(self, u, v):
        self.graph[u].append(v)

    def isReachable(self, s, d):
        visited = [False] * self.V
        queue = []
        visited[s] = True
        queue.append(s)
        while queue:
            node = queue.pop(0)
            if node == d:
                return True
            for j in self.graph[node]:
                if visited[j] == False:
                    visited[j] = True
                    queue.append(j)
        return False


if __name__ == "__main__":
    g = Graph(4)
    g.addEdge(0, 1)
    g.addEdge(0, 2)
    g.addEdge(1, 2)
    g.addEdge(2, 0)
    g.addEdge(2, 3)
    g.addEdge(3, 3)

    u = 1
    v = 3

    if g.isReachable(u, v):
        print("There is a path from %d to %d" % (u, v))
    else:
        print("There is no path from %d to %d" % (u, v))

    u = 3
    v = 1

    if g.isReachable(u, v):
        print("There is a path from %d to %d" % (u, v))
    else:
        print("There is no path from %d to %d" % (u, v))

# There is a path from 1 to 3
# There is no path from 3 to 1