Longest palindrome.

Author: recherst

Easy/405.Convert a Number to Hexadecimal.playground/Contents.swift

@@ -1,4 +1,3 @@
-import Darwin
 /**
  Given an integer num, return a string representing its hexadecimal representation. For negative integers, two’s complement method is used.
 

Easy/409.Longest Palindrome.playground/Contents.swift

@@ -0,0 +1,56 @@
+/**
+ Given a string s which consists of lowercase or uppercase letters, return the length of the longest palindrome that can be built with those letters.
+
+ Letters are case sensitive, for example, "Aa" is not considered a palindrome here.
+
+  
+
+ Example 1:
+ Input: s = "abccccdd"
+ Output: 7
+ Explanation: One longest palindrome that can be built is "dccaccd", whose length is 7.
+ 
+ Example 2:
+ Input: s = "a"
+ Output: 1
+ Explanation: The longest palindrome that can be built is "a", whose length is 1.
+  
+
+ Constraints:
+ - 1 <= s.length <= 2000
+ - s consists of lowercase and/or uppercase English letters only.
+ */
+class Solution {
+    func longestPalindrome(_ s: String) -> Int {
+        if s.count == 1 { return 1 }
+        var map = [Character: Int]()
+        for c in s {
+            if var count = map[c] {
+                count += 1
+                map[c] = count
+            } else {
+                map[c] = 1
+            }
+        }
+        var hasSingle = false
+        map = map.filter { $1 > 1 }
+        var count = 0
+        for e in map {
+            if e.value % 2 == 0 {
+                count += e.value
+            } else {
+                if !hasSingle {
+                    hasSingle = true
+                    count += e.value
+                } else {
+                    count += (e.value - 1)
+                }
+            }
+        }
+        return hasSingle ? count : count + 1 <= s.count ? count + 1 : s.count
+    }
+}
+
+let s = Solution()
+let r = s.longestPalindrome("aabbba")
+print(r)

Easy/409.Longest Palindrome.playground/contents.xcplayground

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<playground version='5.0' target-platform='ios' buildActiveScheme='true' importAppTypes='true'>
+    <timeline fileName='timeline.xctimeline'/>
+</playground>

Easy/409.Longest Palindrome.playground/playground.xcworkspace/contents.xcworkspacedata

@@ -83,6 +83,7 @@
 79. [Binary Watch](https://github.com/recherst/leetcode-algtorithm/blob/main/Easy/401.Binary%20Watch.playground/Contents.swift)
 80. [Sum of Left Leaves](https://github.com/recherst/leetcode-algtorithm/blob/main/Easy/404.Sum%20of%20Left%20Leaves.playground/Contents.swift)
 81. [Convert a Number to Hexadecimal](https://github.com/recherst/leetcode-algtorithm/blob/main/Easy/405.Convert%20a%20Number%20to%20Hexadecimal.playground/Contents.swift)
+82. [Longest Palindrome](https://github.com/recherst/leetcode-algtorithm/blob/main/Easy/409.Longest%20Palindrome.playground/Contents.swift)
 
 #### Medium