three sum

Author: yuzhangcmu

sequence/ThreeSum.java

@@ -1,9 +1,53 @@
 package Algorithms.sequence;
 
+import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.List;
 
 public class ThreeSum {
     public List<List<Integer>> threeSum(int[] num) {
+        List<List<Integer>> ret = new ArrayList<List<Integer>>();
+        if (num == null) {
+            return ret;
+        }
 
+        Arrays.sort(num);
+        
+        int len = num.length;
+        for (int i = 0; i < len; i++) {
+             // 跳过重复的元素，首个数字不需要选择重复的. 
+            if (i > 0 && num[i] == num[i - 1]) {
+                continue;
+            }
+            
+            // 创建二个指针，分别从2头查找，查目标值，它们2个加起来要等于0-num1
+            int l = i + 1;
+            int r = len - 1;
+            while (l < r) {
+                int sum = num[i] + num[l] + num[r];
+                if (sum == 0) {
+                    ArrayList<Integer> list = new ArrayList<Integer>();
+                    list.add(num[i]);
+                    list.add(num[l]);
+                    list.add(num[r]);
+                    ret.add(list);
+                    
+                    // 跳过重复元素 
+                    do {
+                        l++;
+                    } while (l < r && num[l] == num[l - 1]);
+                    
+                    do {
+                        r--;
+                    } while (l < r && num[r] == num[r + 1]);
+                } else if (sum < 0) {
+                    l++;
+                } else {
+                    r--;
+                }
+            }
+        }
+        
+        return ret;
     }
 }