refine

Author: mengli

MedianOfTwoSortedArrays.java

@@ -38,10 +38,4 @@ private double findKth(int A[], int a, int B[], int b, int k) {
 			return A[a + midA - 1];
 		}
 	}
-	
-	public static void main(String[] args) {
-		int[] a = new int[] {1,3,5,6,7,9,10};
-		int[] b = new int[] {2, 4, 8};
-		System.out.println(new MedianOfTwoSortedArrays().findMedianSortedArrays(a, b));
-	}
 }

WordLadderII.java

@@ -1,9 +1,7 @@
-
-
 import java.util.ArrayList;
+import java.util.HashMap;
 import java.util.HashSet;
-import java.util.LinkedList;
-import java.util.Queue;
+import java.util.Map;
 
 /**
  * Given two words (start and end), and a dictionary, find all shortest
@@ -23,63 +21,75 @@
  */
 
 public class WordLadderII {
+	private void GeneratePath(Map<String, ArrayList<String>> prevMap,
+			ArrayList<String> path, String word,
+			ArrayList<ArrayList<String>> ret) {
+		if (prevMap.get(word).size() == 0) {
+			path.add(0, word);
+			ArrayList<String> curPath = new ArrayList<String>(path);
+			ret.add(curPath);
+			path.remove(0);
+			return;
+		}
+
+		path.add(0, word);
+		for (String pt : prevMap.get(word)) {
+			GeneratePath(prevMap, path, pt, ret);
+		}
+		path.remove(0);
+	}
+
 	public ArrayList<ArrayList<String>> findLadders(String start, String end,
 			HashSet<String> dict) {
 		ArrayList<ArrayList<String>> ret = new ArrayList<ArrayList<String>>();
-		int currentLevel = 1, nextLevel = 0;
-		boolean found = false;
-		HashSet<String> visited = new HashSet<String>();
+		Map<String, ArrayList<String>> prevMap = new HashMap<String, ArrayList<String>>();
+		dict.add(start);
+		dict.add(end);
+		for (String d : dict) {
+			prevMap.put(d, new ArrayList<String>());
+		}
+		ArrayList<HashSet<String>> candidates = new ArrayList<HashSet<String>>();
+		candidates.add(new HashSet<String>());
+		candidates.add(new HashSet<String>());
+		int current = 0;
+		int previous = 1;
+		candidates.get(current).add(start);
+		while (true) {
+			current = current == 0 ? 1 : 0;
+			previous = previous == 0 ? 1 : 0;
+			for (String d : candidates.get(previous)) {
+				dict.remove(d);
+			}
+			candidates.get(current).clear();
+			for (String wd : candidates.get(previous)) {
+				for (int pos = 0; pos < wd.length(); ++pos) {
+					StringBuffer word = new StringBuffer(wd);
+					for (int i = 'a'; i <= 'z'; ++i) {
+						if (wd.charAt(pos) == i) {
+							continue;
+						}
 
-		Queue<String> q = new LinkedList<String>();
-		q.add(start);
-		Queue<ArrayList<String>> sequences = new LinkedList<ArrayList<String>>();
-		ArrayList<String> l = new ArrayList<String>();
-		l.add(start);
-		sequences.add(l);
+						word.setCharAt(pos, (char) i);
 
-		while (!q.isEmpty()) {
-			String st = q.remove();
-			ArrayList<String> tmp = sequences.remove();
-			currentLevel--;
-			if (st.equals(end)) {
-				ret.add(tmp);
-				found = true;
-			} else {
-				if (!found) {
-					for (int i = 0; i < st.length(); i++) {
-						StringBuffer sb = new StringBuffer(st);
-						for (int j = 0; j < 26; j++) {
-							sb.setCharAt(i, (char) ('a' + j));
-							String next = sb.toString();
-							boolean in = false;
-							for (int g = 0; g < tmp.size(); g++)
-								if (tmp.get(g).equals(next)) {
-									in = true;
-									break;
-								}
-							if (dict.contains(next) && !in) {
-								q.add(next);
-								visited.add(next);
-								nextLevel++;
-								ArrayList<String> nexttmp = new ArrayList<String>(
-										tmp);
-								nexttmp.add(next);
-								sequences.add(nexttmp);
-							}
+						if (dict.contains(word.toString())) {
+							prevMap.get(word.toString()).add(wd);
+							candidates.get(current).add(word.toString());
 						}
 					}
 				}
 			}
-			if (currentLevel == 0) {
-				if (found)
-					break;
-				else {
-					currentLevel = nextLevel;
-					nextLevel = 0;
-				}
+
+			if (candidates.get(current).size() == 0) {
+				return ret;
+			}
+			if (candidates.get(current).contains(end)) {
+				break;
 			}
 		}
 
+		ArrayList<String> path = new ArrayList<String>();
+		GeneratePath(prevMap, path, end, ret);
+
 		return ret;
 	}
 }