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CyC2018 2018-06-06 21:06:12 +08:00
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notes/算法.md
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@ -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。 ThreeSum 用于统计一个数组中和为 0 的三元组数量。
```java ```java
public class ThreeSum { public interface ThreeSum {
public static int count(int[] nums) { int count(int[] nums);
}
```
```java
public class ThreeSumSlow implements ThreeSum {
@Override
public int count(int[] nums) {
int N = nums.length; int N = nums.length;
int cnt = 0; int cnt = 0;
for (int i = 0; i < N; i++) for (int i = 0; i < N; i++)
@ -76,7 +83,6 @@ public class ThreeSum {
for (int k = j + 1; k < N; k++) for (int k = j + 1; k < N; k++)
if (nums[i] + nums[j] + nums[k] == 0) if (nums[i] + nums[j] + nums[k] == 0)
cnt++; cnt++;
return cnt; return cnt;
} }
} }
@ -112,19 +118,21 @@ public class ThreeSumFast {
``` ```
```java ```java
public class BinarySearch { public class ThreeSumFast implements ThreeSum {
public static int search(int[] nums, int target) { @Override
int l = 0, h = nums.length - 1; public int count(int[] nums) {
while (l <= h) { Arrays.sort(nums);
int m = l + (h - l) / 2; int N = nums.length;
if (target == nums[m]) int cnt = 0;
return m; for (int i = 0; i < N; i++)
else if (target > nums[m]) for (int j = i + 1; j < N; j++) {
l = m + 1; int target = -nums[i] - nums[j];
else int index = BinarySearch.search(nums, target);
h = m - 1; // 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 -1; return cnt;
} }
} }
``` ```
@ -155,7 +163,8 @@ public class RatioTest {
while (loopTimes-- > 0) { while (loopTimes-- > 0) {
int[] nums = new int[N]; int[] nums = new int[N];
StopWatch.start(); StopWatch.start();
int cnt = ThreeSum.count(nums); ThreeSum threeSum = new ThreeSumSlow();
int cnt = threeSum.count(nums);
System.out.println(cnt); System.out.println(cnt);
double elapsedTime = StopWatch.elapsedTime(); double elapsedTime = StopWatch.elapsedTime();
double ratio = preTime == -1 ? 0 : elapsedTime / preTime; double ratio = preTime == -1 ? 0 : elapsedTime / preTime;
@ -225,7 +234,7 @@ public interface MyStack<Item> extends Iterable<Item> {
### 1. 数组实现 ### 1. 数组实现
```java ```java
public class ResizingArrayStack<Item> implements MyStack<Item> { public class ArrayStack<Item> implements MyStack<Item> {
// 栈元素数组 // 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 private Item[] a = (Item[]) new Object[1]; // 只能通过转型来创建泛型数组
// 元素数量 // 元素数量