摘要:題目鏈接檢查圖的連通性及是否有環(huán),可以,,從一個(gè)點(diǎn)出發(fā)看能不能遍歷所有的點(diǎn),同時(shí)來(lái)檢查是否有環(huán)。還可以用檢查是否有環(huán),最后看的數(shù)量是否等于來(lái)判斷是不是。
261. Graph Valid Tree
題目鏈接:https://leetcode.com/problems...
檢查圖的連通性及是否有環(huán),可以dfs,bfs,從一個(gè)點(diǎn)出發(fā)看能不能遍歷所有的點(diǎn),同時(shí)visited來(lái)檢查是否有環(huán)。還可以用union find檢查是否有環(huán),最后看edge的數(shù)量是否等于n-1來(lái)判斷是不是spinning tree。
public class Solution { public boolean validTree(int n, int[][] edges) { if(edges.length != n - 1) return false; map = new int[n]; Arrays.fill(map, -1); // union find for(int[] edge : edges) { int root1 = find(edge[0]); int root2 = find(edge[1]); // if connected before, there is a cycle if(root1 == root2) return false; // union map[root1] = root2; } return true; } int[] map; private int find(int child) { while(map[child] != -1) child = map[child]; return child; } }
dfs注意要保留parent指針,這樣防止出現(xiàn)u->v之后又返回查一遍
public class Solution { public boolean validTree(int n, int[][] edges) { // build graph build(edges, n); // store visited node Setvisited = new HashSet(); // dfs from 0, to check if visited all nodes and if cycle if(dfs(visited, 0, -1)) return false; return visited.size() == n; } // adjacent list to represent graph List > graph; private void build(int[][] edges, int n) { graph = new ArrayList(); for(int i = 0; i < n; i++) graph.add(new HashSet()); for(int[] edge : edges) { graph.get(edge[0]).add(edge[1]); graph.get(edge[1]).add(edge[0]); } } private boolean dfs(Set visited, int u, int parent) { // has cycle if(visited.contains(u)) return true; visited.add(u); for(int v : graph.get(u)) { if(v == parent) continue; if(dfs(visited, v, u)) return true; } return false; } }
bfs同樣要要注意避免走重復(fù)路經(jīng)的問(wèn)題,遍歷過(guò)的點(diǎn)就刪掉。
public class Solution { public boolean validTree(int n, int[][] edges) { // build graph build(edges, n); // store visited node Setvisited = new HashSet(); // bfs from 0, to check if visited all nodes and if cycle return bfs(visited, n); } // adjacent list to represent graph List > graph; private void build(int[][] edges, int n) { graph = new ArrayList(); for(int i = 0; i < n; i++) graph.add(new HashSet()); for(int[] edge : edges) { graph.get(edge[0]).add(edge[1]); graph.get(edge[1]).add(edge[0]); } } private boolean bfs(Set visited, int n) { Queue q = new LinkedList(); q.offer(0); while(!q.isEmpty()) { int u = q.poll(); if(visited.contains(u)) return false; visited.add(u); for(int v : graph.get(u)) { q.offer(v); // remove parent, avoid duplicate graph.get(v).remove(u); } } return visited.size() == n; } }
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