Update mid is l + (r + l) / 2

Author: Desgard

Segment Tree/LazyPropagation/LazyPropagation.playground/Contents.swift

@@ -41,7 +41,7 @@ public class LazySegmentTree {
             return
         }
 
-        let middle = (leftBound + rightBound) / 2
+        let middle = leftBound + (rightBound - leftBound) / 2
         leftChild = LazySegmentTree(array: array, leftBound: leftBound, rightBound: middle)
         rightChild = LazySegmentTree(array: array, leftBound: middle + 1, rightBound: rightBound)
         if let leftChild = leftChild, let rightChild = rightChild {
@@ -62,7 +62,7 @@ public class LazySegmentTree {
 
         pushDown(round: self.rightBound - self.leftBound + 1, lson: leftChild, rson: rightChild)
 
-        let middle = (self.leftBound + self.rightBound) / 2
+        let middle = self.leftBound + (self.rightBound - self.leftBound) / 2
         var result = 0
 
         if leftBound <= middle { result +=  leftChild.query(leftBound: leftBound, rightBound: rightBound) }
@@ -80,7 +80,7 @@ public class LazySegmentTree {
         guard let leftChild  = leftChild  else { fatalError("leftChild should not be nil") }
         guard let rightChild = rightChild else { fatalError("rightChild should not be nil") }
 
-        let middle = (self.leftBound + self.rightBound) / 2
+        let middle = self.leftBound + (self.rightBound - self.leftBound) / 2
 
         if index <= middle { leftChild.update(index: index, incremental: incremental) }
         else { rightChild.update(index: index, incremental: incremental) }
@@ -100,7 +100,7 @@ public class LazySegmentTree {
 
         pushDown(round: self.rightBound - self.leftBound + 1, lson: leftChild, rson: rightChild)
 
-        let middle = (self.leftBound + self.rightBound) / 2
+        let middle = self.leftBound + (self.rightBound - self.leftBound) / 2
 
         if leftBound <= middle { leftChild.update(leftBound: leftBound, rightBound: rightBound, incremental: incremental) }
         if middle < rightBound { rightChild.update(leftBound: leftBound, rightBound: rightBound, incremental: incremental) }
@@ -126,4 +126,4 @@ for index in 2 ... 5 {
 
 
 sumSegmentTree.update(leftBound: 0, rightBound: 5, incremental: 2)
-print(sumSegmentTree.query(leftBound: 0, rightBound: 2))
+print(sumSegmentTree.query(leftBound: 0, rightBound: 2))

Segment Tree/LazyPropagation/README.markdown

@@ -22,6 +22,7 @@ But that is common Segment Tree. By **Lazy Propagation**, we can implement to mo
 At first, we reference the implement of **Artur Antonov** about Segment Tree. This code contained *build*, *single update* and *interval query* three operation. The implement of *build* and *single update* operation is following:
 
 ```swift
+// Author: Artur Antonov
 public init(array: [T], leftBound: Int, rightBound: Int, function: @escaping (T, T) -> T) {
 	self.leftBound = leftBound
 	self.rightBound = rightBound
@@ -45,6 +46,7 @@ In position ①, it means the current node is *leaf* because its left bound data
 And then, we have a look for *interval query* operation:
 
 ```swift
+// Author: Artur Antonov
 public func query(leftBound: Int, rightBound: Int) -> T {
 	if self.leftBound == leftBound && self.rightBound == rightBound {
 		return self.value
@@ -97,7 +99,7 @@ public init(array: [Int], leftBound: Int, rightBound: Int) {
         return
     }
 
-    let middle = (leftBound + rightBound) / 2
+    let middle = leftBound + (rightBound - leftBound) / 2
     leftChild = LazySegmentTree(array: array, leftBound: leftBound, rightBound: middle)
     rightChild = LazySegmentTree(array: array, leftBound: middle + 1, rightBound: rightBound)
     if let leftChild = leftChild, let rightChild = rightChild {
@@ -113,7 +115,7 @@ public func update(index: Int, incremental: Int) {
     guard let leftChild  = leftChild  else { fatalError("leftChild should not be nil") }
     guard let rightChild = rightChild else { fatalError("rightChild should not be nil") }
 
-    let middle = (self.leftBound + self.rightBound) / 2
+    let middle = self.leftBound + (self.rightBound - self.leftBound) / 2
 
     if index <= middle { leftChild.update(index: index, incremental: incremental) }
     else { rightChild.update(index: index, incremental: incremental) }
@@ -198,7 +200,7 @@ public class LazySegmentTree {
             return
         }
 
-        let middle = (leftBound + rightBound) / 2
+        let middle = leftBound + (rightBound - leftBound) / 2
         leftChild = LazySegmentTree(array: array, leftBound: leftBound, rightBound: middle)
         rightChild = LazySegmentTree(array: array, leftBound: middle + 1, rightBound: rightBound)
         if let leftChild = leftChild, let rightChild = rightChild {
@@ -219,7 +221,7 @@ public class LazySegmentTree {
 
         pushDown(round: self.rightBound - self.leftBound + 1, lson: leftChild, rson: rightChild)
 
-        let middle = (self.leftBound + self.rightBound) / 2
+        let middle = self.leftBound + (self.rightBound - self.leftBound) / 2
         var result = 0
 
         if leftBound <= middle { result +=  leftChild.query(leftBound: leftBound, rightBound: rightBound) }
@@ -237,7 +239,7 @@ public class LazySegmentTree {
         guard let leftChild  = leftChild  else { fatalError("leftChild should not be nil") }
         guard let rightChild = rightChild else { fatalError("rightChild should not be nil") }
 
-        let middle = (self.leftBound + self.rightBound) / 2
+        let middle = self.leftBound + (self.rightBound - self.leftBound) / 2
 
         if index <= middle { leftChild.update(index: index, incremental: incremental) }
         else { rightChild.update(index: index, incremental: incremental) }
@@ -257,7 +259,7 @@ public class LazySegmentTree {
 
         pushDown(round: self.rightBound - self.leftBound + 1, lson: leftChild, rson: rightChild)
 
-        let middle = (self.leftBound + self.rightBound) / 2
+        let middle = self.leftBound + (self.rightBound - self.leftBound) / 2
 
         if leftBound <= middle { leftChild.update(leftBound: leftBound, rightBound: rightBound, incremental: incremental) }
         if middle < rightBound { rightChild.update(leftBound: leftBound, rightBound: rightBound, incremental: incremental) }
@@ -302,3 +304,5 @@ In fact, the operation of Segment Tree is far more than that. It can also be use
 ---
 
 *Written for Swift Algorithm Club by [Desgard_Duan](https://github.com/desgard)*
+
+