Merge pull request #139 from qilingit/day27

Add Java solution for day27: Possible Bipartition

Author: jinshendan

May-LeetCoding-Challenge/27-Possible-Bipartition/Possible-Bipartition.java

@@ -0,0 +1,47 @@
+import java.util.HashMap;
+
+class Solution {
+    public boolean possibleBipartition(int N, int[][] dislikes) {
+        /**
+         * Build a graph, try to color each node
+         * 0, If the node hasn't been colored, => color to 1
+         * 1, color its dislikes nodes to - color => color to -1
+         * 2, conflct return false
+         */
+        
+        Map<Integer, Set<Integer>> g = new HashMap<>();
+
+        // build graph
+        for (int[] d : dislikes) {
+            int a = d[0];
+            int b = d[1];
+            g.putIfAbsent(a, new HashSet<Integer>());
+            g.putIfAbsent(b, new HashSet<Integer>());
+            g.get(a).add(b);
+            g.get(b).add(a);
+        }
+
+        int[] colors = new int[N + 1];
+        for (int i = 1; i <= N; i++) {
+            // the node is not colored and there is a conflict
+            if (colors[i] == 0 && !dfs(colors, 1, i, g)) return false;
+        }
+
+        return true;
+    }
+
+    private boolean dfs (int[] colors, int color, int node, Map<Integer, Set<Integer>> g) {
+        // The node is already colored
+        if (colors[node] != 0) {
+            return colors[node] == color;
+        }
+
+        colors[node] = color;
+        if (g.get(node) == null) return true;
+        for (int next : g.get(node)) {
+            if (!dfs(colors, -color, next, g)) return false;
+        }
+
+        return true;
+    }
+}