modify code on class

Author: algorithmzuo

src/class02/Code01_PreSum.java

@@ -29,7 +29,7 @@ public RangeSum2(int[] array) {
 			preSum = new int[N];
 			preSum[0] = array[0];
 			for (int i = 1; i < N; i++) {
-				preSum[i] = preSum[i + 1] + array[i];
+				preSum[i] = preSum[i - 1] + array[i];
 			}
 		}
 

src/class02/Code02_RandToRand.java

@@ -104,31 +104,128 @@ public static int random(RandomBox randomBox, int from, int to) {
 	}
 
 	public static void main(String[] args) {
+		System.out.println("测试开始");
+		// Math.random() -> double -> [0,1)
+		//
+
+		int testTimes = 10000000;
+		int count = 0;
+		for (int i = 0; i < testTimes; i++) {
+			if (Math.random() < 0.75) {
+				count++;
+			}
+		}
+		System.out.println((double) count / (double) testTimes);
+
+		System.out.println("=========");
+
+		// [0,1) -> [0,8)
+		count = 0;
+		for (int i = 0; i < testTimes; i++) {
+			if (Math.random() * 8 < 5) {
+				count++;
+			}
+		}
+		System.out.println((double) count / (double) testTimes);
+		System.out.println((double) 5 / (double) 8);
+
+		int K = 9;
+		// [0,K) -> [0,8]
+
+		int[] counts = new int[9];
+		for (int i = 0; i < testTimes; i++) {
+			int ans = (int) (Math.random() * K); // [0,K-1]
+			counts[ans]++;
+		}
+		for (int i = 0; i < K; i++) {
+			System.out.println(i + "这个数，出现了 " + counts[i] + " 次");
+		}
+
+		System.out.println("=========");
 
-		int[] count = new int[8]; // 0 1 2 .. 7
-		int testTime = 10000000;
-		for (int i = 0; i < testTime; i++) {
-			int ans = c();
-			count[ans]++;
+		count = 0;
+		double x = 0.17;
+		for (int i = 0; i < testTimes; i++) {
+			if (xToXPower2() < x) {
+				count++;
+			}
+		}
+		System.out.println((double) count / (double) testTimes);
+		System.out.println((double) 1 - Math.pow((double) 1 - x, 2));
+
+		System.out.println("==========");
+		count = 0;
+		for (int i = 0; i < testTimes; i++) {
+			if (f2() == 0) {
+				count++;
+			}
 		}
+		System.out.println((double) count / (double) testTimes);
 
+		System.out.println("==========");
+
+		counts = new int[8];
+		for (int i = 0; i < testTimes; i++) {
+			int num = g();
+			counts[num]++;
+		}
 		for (int i = 0; i < 8; i++) {
-			System.out.println(i + " : " + count[i]);
+			System.out.println(i + "这个数，出现了 " + counts[i] + " 次");
 		}
 
-//		RandomBox randomBox = new RandomBox(3, 9);
-//		int from = 17;
-//		int to = 29;
-//		int[] ans = new int[to + 1];
-//		int testTime1 = 1000000;
-//		for (int i = 0; i < testTime1; i++) {
-//			ans[random(randomBox, from, to)]++;
-//		}
-//		for (int i = 0; i < ans.length; i++) {
-//			System.out.println(ans[i]);
-//		}
-//		System.out.println("==========");
+	}
+
+	// 返回[0,1)的一个小数
+	// 任意的x，x属于[0,1)，[0,x)范围上的数出现概率由原来的x调整成x平方
+	public static double xToXPower2() {
+		return Math.min(Math.random(), Math.random());
+	}
+
+	// lib里的，不能改！
+	public static int f1() {
+		return (int) (Math.random() * 5) + 1;
+	}
 
+	// 随机机制，只能用f1，
+	// 等概率返回0和1
+	public static int f2() {
+		int ans = 0;
+		do {
+			ans = f1();
+		} while (ans == 3);
+		return ans < 3 ? 0 : 1;
+	}
+
+	// 得到000 ~ 111 做到等概率 0 ~ 7等概率返回一个
+	public static int f3() {
+		return (f2() << 2) + (f2() << 1) + f2();
+	}
+
+	// 0 ~ 6等概率返回一个
+	public static int f4() {
+		int ans = 0;
+		do {
+			ans = f3();
+		} while (ans == 7);
+		return ans;
+	}
+
+	public static int g() {
+		return f4() + 1;
+	}
+
+	// 你只能知道，x会以固定概率返回0和1，但是x的内容，你看不到！
+	public static int x() {
+		return Math.random() < 0.84 ? 0 : 1;
+	}
+
+	// 等概率返回0和1
+	public static int y() {
+		int ans = 0;
+		do {
+			ans = x();
+		} while (ans == x());
+		return ans;
 	}
 
 }

src/class02/Code03_Comp.java

@@ -0,0 +1,90 @@
+package class02;
+
+public class Code03_Comp {
+
+	public static void selectionSort(int[] arr) {
+		if (arr == null || arr.length < 2) {
+			return;
+		}
+		for (int i = 0; i < arr.length - 1; i++) {
+			int minIndex = i;
+			for (int j = i + 1; j < arr.length; j++) {
+				if (arr[j] < arr[minIndex]) {
+					minIndex = j;
+				}
+			}
+			swap(arr, i, minIndex);
+		}
+	}
+
+	public static void swap(int[] arr, int i, int j) {
+		int tmp = arr[i];
+		arr[i] = arr[j];
+		arr[j] = tmp;
+	}
+
+	public static void insertionSort(int[] arr) {
+		if (arr == null || arr.length < 2) {
+			return;
+		}
+		for (int i = 1; i < arr.length; i++) { // 0 ~ i 做到有序
+			for (int j = i - 1; j >= 0 && arr[j] > arr[j + 1]; j--) {
+				swap(arr, j, j + 1);
+			}
+		}
+	}
+
+	// 返回一个数组arr，arr长度[0,maxLen-1],arr中的每个值[0,maxValue-1]
+	public static int[] lenRandomValueRandom(int maxLen, int maxValue) {
+		int len = (int) (Math.random() * maxLen);
+		int[] ans = new int[len];
+		for (int i = 0; i < len; i++) {
+			ans[i] = (int) (Math.random() * maxValue);
+		}
+		return ans;
+	}
+
+	public static int[] copyArray(int[] arr) {
+		int[] ans = new int[arr.length];
+		for (int i = 0; i < arr.length; i++) {
+			ans[i] = arr[i];
+		}
+		return ans;
+	}
+
+	// arr1和arr2一定等长
+	public static boolean isSorted(int[] arr) {
+		if (arr.length < 2) {
+			return true;
+		}
+		int max = arr[0];
+		for (int i = 1; i < arr.length; i++) {
+			if (max > arr[i]) {
+				return false;
+			}
+			max = Math.max(max, arr[i]);
+		}
+		return true;
+	}
+
+	public static void main(String[] args) {
+		int maxLen = 5;
+		int maxValue = 1000;
+		int testTime = 10000;
+		for (int i = 0; i < testTime; i++) {
+			int[] arr1 = lenRandomValueRandom(maxLen, maxValue);
+			int[] tmp = copyArray(arr1);
+			selectionSort(arr1);
+			if (!isSorted(arr1)) {
+				for (int j = 0; j < tmp.length; j++) {
+					System.out.print(tmp[j] + " ");
+				}
+				System.out.println();
+				System.out.println("选择排序错了！");
+				break;
+			}
+		}
+
+	}
+
+}