feat: solved 15

Author: fghpdf

src/com/fghpdf/ThreeSum/Solution.java

@@ -0,0 +1,50 @@
+package com.fghpdf.ThreeSum;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+
+/**
+ * @author fghpdf
+ * @date 2019/11/11
+ * https://leetcode.com/problems/3sum/
+ *
+ * can't to loop 3 times, it will overtime
+ * first sort
+ * then loop 1 time and to compute 3 numbers
+ * if sum == 0, should eliminate same numbers
+ * if sum < 0, lower number is too small
+ * if sum > 0, higher number is too big
+ **/
+public class Solution {
+	public List<List<Integer>> threeSum(int[] nums) {
+		Arrays.sort(nums);
+
+		List<List<Integer>> result = new ArrayList<>();
+		for (int i = 0; i < nums.length - 2; i++) {
+			if (i == 0 || nums[i] != nums[i - 1]) {
+				int lower = i + 1;
+				int higher = nums.length - 1;
+				while (lower < higher) {
+					if (nums[i] + nums[lower] + nums[higher] == 0) {
+						result.add(Arrays.asList(nums[i], nums[lower], nums[higher]));
+						// to eliminate same number
+						while (lower < higher && nums[lower] == nums[lower+1]) {
+							lower++;
+						}
+						while (lower < higher && nums[higher] == nums[higher-1]) {
+							higher--;
+						}
+						lower++;
+						higher--;
+					} else if (nums[lower] + nums[higher] + nums[i] < 0) {
+						lower++;
+					} else {
+						higher--;
+					}
+				}
+			}
+		}
+		return result;
+	}
+}