15. 三数之和

Author: geekidentity

src/main/java/com/geekidentity/leetcode/n0015/ThreeSum.java

@@ -0,0 +1,55 @@
+package com.geekidentity.leetcode.n0015;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+
+/**
+ * 15. 三数之和
+ * https://leetcode-cn.com/problems/3sum/
+ * 1. 暴击
+ * 2. 双指针，左右下标向中间推进
+ */
+public class ThreeSum {
+    public List<List<Integer>> threeSum1(int[] nums) {
+        List<List<Integer>> result = new ArrayList<>();
+        for (int i = 0; i < nums.length - 2; i++) {
+            for (int j = i + 1; j < nums.length - 1; j++) {
+                for (int k = j + 1; k < nums.length; k++) {
+                    if (nums[i] + nums[j] + nums[k] == 0) {
+                        result.add(Arrays.asList(nums[i], nums[j], nums[k]));
+                    }
+                }
+            }
+        }
+        return result;
+    }
+
+    public List<List<Integer>> threeSum2(int[] nums) {
+        List<List<Integer>> result = new ArrayList<>();
+        Arrays.sort(nums);
+        for (int i = 0; i < nums.length - 2; i++) {
+            if (i > 0 && nums[i] == nums[i - 1]) continue;
+            for (int j = i + 1, k = nums.length - 1; j < k;) {
+                int target = -nums[i];
+                int sum = nums[j] + nums[k];
+                if (sum == target) {
+                    result.add(Arrays.asList(nums[i], nums[j], nums[k]));
+                    j++;
+                    k--;
+                    while (j < k && nums[j] == nums[j - 1]) {
+                        j++;
+                    }
+                    while (j < k && nums[k] == nums[k + 1]) {
+                        k--;
+                    }
+                } else if (sum < target) {
+                    j++;
+                } else {
+                    k--;
+                }
+            }
+        }
+        return result;
+    }
+}