From c21781366eaffb729178f08844ca105ba1e5dd8d Mon Sep 17 00:00:00 2001
From: huangge1199 <huangge1199@hotmail.com>
Date: Tue, 10 Aug 2021 16:19:00 +0800
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 .../cn/LongestArithmeticSubsequence.java      | 80 +++++++++++++++++++
 .../editor/cn/LongestArithmeticSubsequence.md | 39 +++++++++
 2 files changed, 119 insertions(+)
 create mode 100644 src/main/java/leetcode/editor/cn/LongestArithmeticSubsequence.java
 create mode 100644 src/main/java/leetcode/editor/cn/LongestArithmeticSubsequence.md

diff --git a/src/main/java/leetcode/editor/cn/LongestArithmeticSubsequence.java b/src/main/java/leetcode/editor/cn/LongestArithmeticSubsequence.java
new file mode 100644
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+++ b/src/main/java/leetcode/editor/cn/LongestArithmeticSubsequence.java
@@ -0,0 +1,80 @@
+//给定一个整数数组 A，返回 A 中最长等差子序列的长度。 
+//
+// 回想一下，A 的子序列是列表 A[i_1], A[i_2], ..., A[i_k] 其中 0 <= i_1 < i_2 < ... < i_k <= A
+//.length - 1。并且如果 B[i+1] - B[i]( 0 <= i < B.length - 1) 的值都相同，那么序列 B 是等差的。 
+//
+// 
+//
+// 示例 1： 
+//
+// 输入：[3,6,9,12]
+//输出：4
+//解释： 
+//整个数组是公差为 3 的等差数列。
+// 
+//
+// 示例 2： 
+//
+// 输入：[9,4,7,2,10]
+//输出：3
+//解释：
+//最长的等差子序列是 [4,7,10]。
+// 
+//
+// 示例 3： 
+//
+// 输入：[20,1,15,3,10,5,8]
+//输出：4
+//解释：
+//最长的等差子序列是 [20,15,10,5]。
+// 
+//
+// 
+//
+// 提示： 
+//
+// 
+// 2 <= A.length <= 2000 
+// 0 <= A[i] <= 10000 
+// 
+// Related Topics 数组 哈希表 二分查找 动态规划 
+// 👍 144 👎 0
+
+package leetcode.editor.cn;
+
+import java.util.Arrays;
+
+//1027:最长等差数列
+class LongestArithmeticSubsequence {
+    public static void main(String[] args) {
+        //测试代码
+        Solution solution = new LongestArithmeticSubsequence().new Solution();
+    }
+
+    //力扣代码
+    //leetcode submit region begin(Prohibit modification and deletion)
+    class Solution {
+        public int longestArithSeqLength(int[] nums) {
+            int max = 0;
+            for (int num : nums) {
+                max = Math.max(max, num);
+            }
+            int[][] dp = new int[nums.length][max*2+1];
+            for (int[] t:dp){
+                Arrays.fill(t,-1);
+            }
+
+            int result = 2;
+            for (int i = 0; i < nums.length; i++) {
+                for (int j = 0; j < i; j++) {
+                    int index = nums[i]-nums[j]+max;
+                    dp[i][index]=Math.max(2,dp[j][index]+1);
+                    result = Math.max(result,dp[i][index]);
+                }
+            }
+            return result;
+        }
+    }
+//leetcode submit region end(Prohibit modification and deletion)
+
+}
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diff --git a/src/main/java/leetcode/editor/cn/LongestArithmeticSubsequence.md b/src/main/java/leetcode/editor/cn/LongestArithmeticSubsequence.md
new file mode 100644
index 0000000..4f32908
--- /dev/null
+++ b/src/main/java/leetcode/editor/cn/LongestArithmeticSubsequence.md
@@ -0,0 +1,39 @@
+<p>给定一个整数数组&nbsp;<code>A</code>，返回 <code>A</code>&nbsp;中最长等差子序列的<strong>长度</strong>。</p>
+
+<p>回想一下，<code>A</code>&nbsp;的子序列是列表&nbsp;<code>A[i_1], A[i_2], ..., A[i_k]</code> 其中&nbsp;<code>0 &lt;= i_1 &lt; i_2 &lt; ... &lt; i_k &lt;= A.length - 1</code>。并且如果&nbsp;<code>B[i+1] - B[i]</code>(&nbsp;<code>0 &lt;= i &lt; B.length - 1</code>) 的值都相同，那么序列&nbsp;<code>B</code>&nbsp;是等差的。</p>
+
+<p>&nbsp;</p>
+
+<p><strong>示例 1：</strong></p>
+
+<pre><strong>输入：</strong>[3,6,9,12]
+<strong>输出：</strong>4
+<strong>解释： </strong>
+整个数组是公差为 3 的等差数列。
+</pre>
+
+<p><strong>示例 2：</strong></p>
+
+<pre><strong>输入：</strong>[9,4,7,2,10]
+<strong>输出：</strong>3
+<strong>解释：</strong>
+最长的等差子序列是 [4,7,10]。
+</pre>
+
+<p><strong>示例 3：</strong></p>
+
+<pre><strong>输入：</strong>[20,1,15,3,10,5,8]
+<strong>输出：</strong>4
+<strong>解释：</strong>
+最长的等差子序列是 [20,15,10,5]。
+</pre>
+
+<p>&nbsp;</p>
+
+<p><strong>提示：</strong></p>
+
+<ol>
+	<li><code>2 &lt;= A.length &lt;= 2000</code></li>
+	<li><code>0 &lt;= A[i] &lt;= 10000</code></li>
+</ol>
+<div><div>Related Topics</div><div><li>数组</li><li>哈希表</li><li>二分查找</li><li>动态规划</li></div></div>\n<div><li>👍 144</li><li>👎 0</li></div>
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