feat: 优化归并排序性能

.vscode/launch.json

@@ -0,0 +1,17 @@
+{
+  // Use IntelliSense to learn about possible attributes.
+  // Hover to view descriptions of existing attributes.
+  // For more information, visit: https://go.microsoft.com/fwlink/?linkid=830387
+  "version": "0.2.0",
+  "configurations": [
+    {
+      "type": "node",
+      "request": "launch",
+      "name": "Launch Program",
+      "program": "${workspaceFolder}/src/algs4/sort/merge-sort.ts",
+      "outFiles": ["${workspaceFolder}/dist/**/*.js"]
+    }
+  ]
+}
+
+

src/algs4/sort/base-sort.ts

@@ -7,7 +7,7 @@ abstract class BaseSort<T> {
     this.array = array
   }
 
-  abstract sort(): void
+  abstract sort(array?: T[]): void
 
   protected less(v: T, w: T): boolean {
     return v < w
@@ -30,8 +30,8 @@ abstract class BaseSort<T> {
     return true
   }
 
-  main() {
-    this.sort()
+  main(passArray?: boolean) {
+    passArray ? this.sort(this.array) : this.sort()
     assert(this.isSorted(), '数组排序错误')
     this.show()
   }

src/algs4/sort/merge-sort.ts

@@ -3,42 +3,41 @@ import { StdIn, StopWatch } from 'utils'
 import { SortMockFile } from '@/constants'
 
 class MergeSort<T> extends BaseSort<T> {
-  sort() {
-    this.array = this._sort(this.array)
+  private aux: T[] // 归并所需的辅助数组
+
+  sort(a: T[]) {
+    this.aux = []
+    this._sort(a, 0, a.length - 1)
   }
 
-  private _sort(array: T[]): T[] {
-    if (array.length <= 1) return array
-    const middle = Math.floor(array.length / 2)
-    const left = array.slice(0, middle)
-    const right = array.slice(middle)
-    return this.merge(this._sort(left), this._sort(right))
+  private _sort(a: T[], lo: number, hi: number) {
+    if (hi <= lo) return
+    const mid = lo + Math.floor((hi - lo) / 2)
+    this._sort(a, lo, mid) // 将左半边排序
+    this._sort(a, mid + 1, hi) // 将右半边排序
+    this.merge(a, lo, mid, hi) // 归并结果
   }
 
-  private merge(left: T[], right: T[]): T[] {
-    const res: T[] = []
-    while (left.length && right.length) {
-      if (left[0] < right[0]) {
-        res.push(left.shift())
-      } else {
-        res.push(right.shift())
-      }
-    }
-    while (left.length) {
-      res.push(left.shift())
+  private merge(a: T[], lo: number, mid: number, hi: number) {
+    // 将a[lo..mid] 和 a[mid+1..hi] 归并
+    let i = lo; let j = mid + 1
+    for (let k = lo; k <= hi; k++) { // 将a[lo..mid] 复制到 aux[lo..hi]
+      this.aux[k] = a[k]
     }
-    while (right.length) {
-      res.push(right.shift())
+    for (let k = lo; k <= hi; k++) { // 归并回到 a[lo..hi]
+      if (i > mid) a[k] = this.aux[j++]
+      else if (j > hi) a[k] = this.aux[i++]
+      else if (this.less(this.aux[j], this.aux[i])) a[k] = this.aux[j++]
+      else a[k] = this.aux[i++]
     }
-    return res
   }
 }
 
 async function main() {
   const data = await StdIn.readInt(SortMockFile)
   const stopWatch = new StopWatch()
   const merge = new MergeSort<number>(data)
-  merge.main()
+  merge.main(true)
   stopWatch.elapseTime()
 }
 

tsconfig.json

@@ -10,7 +10,8 @@
       "@/*": ["*"],
       "utils": ["utils"],
       "mock": ["mock"]
-    }
+    },
+    "sourceMap": true
   },
   "include": ["src/**/*"],
   "exclude": ["node_modules", "**/*.spec.ts"]

webpack.config.js

@@ -53,6 +53,7 @@ module.exports = {
     },
     extensions: ['.ts', '.js']
   },
+  devtool: 'source-map',
   devServer: {
     contentBase: './dist'
   },