From c392edcc1376dfa83e07c0eed10841cab6566035 Mon Sep 17 00:00:00 2001
From: keineahnung2345 <mimifasosofamire1123@gmail.com>
Date: Sun, 25 Oct 2020 09:51:22 +0800
Subject: [PATCH] O(n) nth_element

---
 ... of Array After Removing Some Elements.cpp | 22 +++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/1619. Mean of Array After Removing Some Elements.cpp b/1619. Mean of Array After Removing Some Elements.cpp
index 865c5ba..8ffcdd2 100644
--- a/1619. Mean of Array After Removing Some Elements.cpp	
+++ b/1619. Mean of Array After Removing Some Elements.cpp	
@@ -12,3 +12,25 @@ class Solution {
         return (double)accumulate(arr.begin()+rem, arr.end()-rem, 0)/(n-2*rem);
     }
 };
+
+//O(n) nth_element
+//https://leetcode.com/problems/mean-of-array-after-removing-some-elements/discuss/900529/C%2B%2B-O(n)-nth_element
+//Runtime: 20 ms, faster than 45.74% of C++ online submissions for Mean of Array After Removing Some Elements.
+//Memory Usage: 9.8 MB, less than 99.83% of C++ online submissions for Mean of Array After Removing Some Elements.
+class Solution {
+public:
+    double trimMean(vector<int>& arr) {
+        int n = arr.size();
+        
+        int rem = n*5.0/100;
+        
+        //[rem, n-rem*2, rem]
+        
+        //put the first "rem" elements at their right place
+        nth_element(arr.begin(), arr.begin()+rem, arr.end());
+        //put the second part(n-rem*2 elements) at their right place
+        nth_element(arr.begin()+rem, arr.end()-rem, arr.end());
+        
+        return accumulate(arr.begin()+rem, arr.end()-rem, 0)/(double)(n-rem*2);
+    }
+};