auto commit

notes/算法.md

@@ -67,8 +67,15 @@ N<sup>3</sup>/6-N<sup>2</sup>/2+N/3 的增长数量级为 O(N<sup>3</sup>)。增
 ThreeSum 用于统计一个数组中和为 0 的三元组数量。
 
 ```java
-public class ThreeSum {
-    public static int count(int[] nums) {
+public interface ThreeSum {
+    int count(int[] nums);
+}
+```
+
+```java
+public class ThreeSumSlow implements ThreeSum {
+    @Override
+    public int count(int[] nums) {
         int N = nums.length;
         int cnt = 0;
         for (int i = 0; i < N; i++)
@@ -76,7 +83,6 @@ public class ThreeSum {
                 for (int k = j + 1; k < N; k++)
                     if (nums[i] + nums[j] + nums[k] == 0)
                         cnt++;
-
         return cnt;
     }
 }
@@ -112,19 +118,21 @@ public class ThreeSumFast {
 ```
 
 ```java
-public class BinarySearch {
-    public static int search(int[] nums, int target) {
-        int l = 0, h = nums.length - 1;
-        while (l <= h) {
-            int m = l + (h - l) / 2;
-            if (target == nums[m])
-                return m;
-            else if (target > nums[m])
-                l = m + 1;
-            else
-                h = m - 1;
-        }
-        return -1;
+public class ThreeSumFast implements ThreeSum {
+    @Override
+    public int count(int[] nums) {
+        Arrays.sort(nums);
+        int N = nums.length;
+        int cnt = 0;
+        for (int i = 0; i < N; i++)
+            for (int j = i + 1; j < N; j++) {
+                int target = -nums[i] - nums[j];
+                int index = BinarySearch.search(nums, target);
+                // 应该注意这里的下标必须大于 j，否则会重复统计。
+                if (index > j)
+                    cnt++;
+            }
+        return cnt;
     }
 }
 ```
@@ -155,7 +163,8 @@ public class RatioTest {
         while (loopTimes-- > 0) {
             int[] nums = new int[N];
             StopWatch.start();
-            int cnt = ThreeSum.count(nums);
+            ThreeSum threeSum = new ThreeSumSlow();
+            int cnt = threeSum.count(nums);
             System.out.println(cnt);
             double elapsedTime = StopWatch.elapsedTime();
             double ratio = preTime == -1 ? 0 : elapsedTime / preTime;
@@ -225,7 +234,7 @@ public interface MyStack<Item> extends Iterable<Item> {
 ### 1. 数组实现
 
 ```java
-public class ResizingArrayStack<Item> implements MyStack<Item> {
+public class ArrayStack<Item> implements MyStack<Item> {
     // 栈元素数组
     private Item[] a = (Item[]) new Object[1]; // 只能通过转型来创建泛型数组
     // 元素数量