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Author: assassint2017

Is Graph Bipartite/Is_Graph_Bipartite.cpp

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+/*
+
+附上大佬的参考代码
+https://leetcode.com/problems/is-graph-bipartite/discuss/115487/Java-Clean-DFS-solution-with-Explanation
+
+u 代表还没有被上色
+r 表示红色
+b 表示蓝色
+*/
+
+// 单纯使用简单的DFS的代码有点超时，还需要再优化一下
+// 63 / 78 test cases passed.
+class Solution 
+{
+public:
+    bool isBipartite(vector<vector<int>>& graph) 
+    {
+        if (graph.size() == 0) return true;
+        
+        vector<char> memo(graph.size(), 'u');
+        
+        return safe(graph, memo, 0);
+    }
+private:
+    bool safe(const vector<vector<int>>& graph, vector<char>& memo, int startIndex)
+    {
+        // 递归停止条件
+        if (startIndex == graph.size()) return true;
+        
+        if (memo[startIndex] != 'u')
+        {
+            for (auto node : graph[startIndex])
+            {
+                if (memo[node] == memo[startIndex]) return false;
+            }
+
+            char color = memo[startIndex] == 'b' ? 'r' : 'b';
+            for (auto node : graph[startIndex])
+            {
+                memo[node] = color;
+            }
+            
+            return safe(graph, memo, startIndex + 1);
+        }
+        else
+        {
+            vector<char> tempmemo = memo;
+
+            vector<char> colors = {'b', 'r'};
+            for (char color : colors)
+            {
+                memo[startIndex] = color;
+
+                bool bad = false;
+                for (auto node : graph[startIndex])
+                {
+                    if (memo[node] == memo[startIndex])
+                    {
+                        bad = true;
+                        break;
+                    }
+                }
+
+                if (bad) continue;
+
+                char tempcolor = memo[startIndex] == 'b' ? 'r' : 'b';
+                for (auto node : graph[startIndex])
+                {
+                    memo[node] = tempcolor;
+                }
+
+                if (safe(graph, memo, startIndex + 1)) 
+                    return true;
+                else
+                    memo = tempmemo;
+            }
+
+            memo[startIndex] = 'u';
+            return false;
+        }    
+    }
+};
+
+// 充分理解题目意思之后的BFS改进代码
+// Runtime: 44 ms, faster than 7.69% of C++ online submissions for Is Graph Bipartite?.
+// Memory Usage: 11.9 MB, less than 38.46% of C++ online submissions for Is Graph Bipartite?.
+class Solution 
+{
+public:
+    bool isBipartite(vector<vector<int>>& graph) 
+    {
+        vector<char> memo(graph.size(), 'u');
+        
+        for (int index = 0; index < memo.size(); ++index)
+        {
+            if (memo[index] != 'u') continue;
+
+            queue<int> q;
+            int layer = 1;
+            q.push(index);
+
+            while (!q.empty())
+            {
+                for (int i = q.size(); i > 0; --i)
+                {
+                    int node = q.front();
+                    memo[node] = layer % 2 == 0 ? 'r' : 'b';
+                    q.pop();
+
+                    for (auto nextNode : graph[node])
+                    {
+                        if (memo[node] == memo[nextNode]) return false;
+                    }
+
+                    for (auto nextNode : graph[node])
+                    {
+                        if (memo[nextNode] == 'u') q.push(nextNode);
+                    }
+                }
+                ++layer;
+            }    
+        }
+        return true;
+    }
+};