fixed small bug in quick select algorithm (rust-lang#727)

Author: fer2o3

src/searching/quick_select.rs

@@ -4,13 +4,13 @@ fn partition(list: &mut [i32], left: usize, right: usize, pivot_index: usize) ->
     let pivot_value = list[pivot_index];
     list.swap(pivot_index, right); // Move pivot to end
     let mut store_index = left;
-    for i in left..(right + 1) {
+    for i in left..right {
         if list[i] < pivot_value {
             list.swap(store_index, i);
             store_index += 1;
         }
-        list.swap(right, store_index); // Move pivot to its final place
     }
+    list.swap(right, store_index); // Move pivot to its final place
     store_index
 }
 
@@ -19,7 +19,7 @@ pub fn quick_select(list: &mut [i32], left: usize, right: usize, index: usize) -
         // If the list contains only one element,
         return list[left];
     } // return that element
-    let mut pivot_index = 1 + left + (right - left) / 2; // select a pivotIndex between left and right
+    let mut pivot_index = left + (right - left) / 2; // select a pivotIndex between left and right
     pivot_index = partition(list, left, right, pivot_index);
     // The pivot is in its final sorted position
     match index {
@@ -37,7 +37,7 @@ mod tests {
         let mut arr1 = [2, 3, 4, 5];
         assert_eq!(quick_select(&mut arr1, 0, 3, 1), 3);
         let mut arr2 = [2, 5, 9, 12, 16];
-        assert_eq!(quick_select(&mut arr2, 1, 3, 2), 12);
+        assert_eq!(quick_select(&mut arr2, 1, 3, 2), 9);
         let mut arr2 = [0, 3, 8];
         assert_eq!(quick_select(&mut arr2, 0, 0, 0), 0);
     }