First commit - add a Java Folder

Author: sfatdisk

Java/3Sum.java

@@ -0,0 +1,38 @@
+/*
+Note: 
+sort,nums[i]<=0, remove duplicates
+*/
+
+
+public class Solution {
+    public List<List<Integer>> threeSum(int[] nums) {
+        Arrays.sort(nums);
+        List< List<Integer> > results= new ArrayList< List<Integer> >(); 
+        
+        for(int i=0;i< nums.length-2 && nums[i]<=0 ;++i){
+            int l= i+1;
+            int r= nums.length-1;
+            if(i>0 && nums[i]==nums[i-1]){continue;}
+            
+            while(l<r){
+                if(nums[l]+nums[i]+nums[r]==0){
+                    List<Integer> result= new ArrayList<Integer>();
+                    result.add(nums[i]);
+                    result.add(nums[l]);
+                    result.add(nums[r]);
+                    results.add(result);
+                    l++;
+                    r--;
+                    while(l<r && nums[l]==nums[l-1]) { l++; }
+                    while(l<r && nums[r]==nums[r+1]) { r--; }
+                }else if(nums[l]+nums[r]+nums[i]<0){
+                    l++;
+                }else{
+                    r--;
+                }
+            }
+            
+        }
+        return results;
+    }
+}

Java/4Sum.java

@@ -0,0 +1,39 @@
+/*
+*/
+
+public class Solution {
+    public List<List<Integer>> fourSum(int[] nums, int target) {
+        
+        List<List<Integer> > results= new ArrayList<List<Integer> >();
+        Arrays.sort(nums);
+        for(int i=0; i< nums.length-3 ;i++){
+            if(i>0 && nums[i]==nums[i-1]){continue;}
+            // Three sum
+            for(int j=i+1; j< nums.length-2 ;j++ ){
+                if( j>i+1 && nums[j]==nums[j-1] ){ continue; }
+                int l= j+1;
+                int r= nums.length-1;
+                
+                while(l<r){
+                    if(nums[i]+nums[j]+nums[l]+nums[r] >target ){
+                        r--;
+                    }else if(nums[i]+nums[j]+nums[l]+nums[r]<target){
+                        l++;
+                    }else{
+                        List<Integer> result = new ArrayList<Integer>();
+                        result.add(nums[i]);
+                        result.add(nums[j]);
+                        result.add(nums[l]);
+                        result.add(nums[r]);
+                        results.add(result);
+                        l++;
+                        r--;
+                        while( l<r && nums[l-1]==nums[l]){ l++; }
+                        while( l<r && nums[r+1]==nums[r]){ r--; }
+                    }
+                }
+            }
+        }
+        return results; 
+    }
+}

Java/ReverseLinkList.java

@@ -0,0 +1,40 @@
+/* 
+check at least 2 nodes
+setup pre and current nodes
+use next node to keep the safe place 
+when we moving nodes to next place. 
+*/
+
+public void reverseLinkList( Node head){
+	if( head ==null || head.next==null ){
+		return;
+	}
+
+	Node pre= null;
+	Node current= head; 
+	while( current !=null ){
+		Node next= current.next; 
+		current.next=pre;
+		pre= current; 
+		current= next;
+	}
+	head= pre;
+}	
+
+//  http://www.programmerinterview.com/index.php/data-structures/reverse-a-linked-list/
+ 
+public void reverseLinkList(Node current){
+	
+	// for empty list
+	if(current ==null){ return; }
+
+	// stop case
+	if(current.next == null ){
+		Node head= current;
+		return;
+	}
+	reverseLinkList(current.next);
+	current.next.next= current; 
+	current.next=null;
+}
+

Java/TwoSum.java

@@ -0,0 +1,45 @@
+/* HasMap idea:
+ * if lookingFor is not in the HashMap
+ *    store current number in the HasMap
+ * else
+ *    return the nums[]
+ *
+ */
+
+// 1
+public class Solution {
+    public int[] twoSum(int[] nums, int target) {
+        
+        HashMap <Integer,Integer> map= new HashMap();
+        for( int i=0;i< nums.length ;++i ){
+            int lookingFor= target-nums[i];
+            if( map.get(lookingFor)!=null ){
+                return new int[]{ map.get(lookingFor), i+1 }; 
+            }else{
+                map.put( nums[i], i+1);
+            }
+        }
+        return null;
+    }
+}
+
+// 167 Two Sum II - Input array is sorted
+public class Solution {
+    public int[] twoSum(int[] nums, int target) {
+        int[] result = new int[2];
+        int l=0;
+        int r=nums.length;
+        while(l<r){
+            if(nums[l]+nums[r]==target){
+                result[0]=nums[l];
+                result[1]=nums[r];
+                retrun result; 
+            }else if(nums[l]+nums[r]>target){
+                r--;
+            }else{
+                l++;
+            }   
+        }
+        retur null;
+    }
+}