排序和查找

交换函数

public static void swap(int[] arr, int i, int j) {
    int temp = arr[i];
    arr[i] = arr[j];
    arr[j] = temp;
}

1. 冒泡排序

public static void bubbleSort(int[] arr) {
    for(int i = 0; i < arr.length - 1; i++) {
        for(int j = 0; j < arr.length - i - 1; j++) {
            if(arr[j] > arr[j + 1]) {
                swap(arr, j, j + 1);
            }
        }
    }
}

2. 选择排序

public static void selectSort(int[] arr) {
    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);
    }
}

3. 快速排序

public static void quickSort(int[] arr, int start, int end) {
    if(start < end) {
        int pivot = arr[start];
        int low = start;
        int high = end + 1;
        while (true) {
            while (low < end && arr[++low] <= pivot);
            while (high > start && arr[--high] > pivot);
            if(low < high)
                swap(arr, low, high);
            else
                break;
        }
        swap(arr, start, high);
        quickSort(arr, start, high - 1);
        quickSort(arr, high + 1, end);
    }
}

4.插入排序

public static void insertSort(int[] arr) {
    for(int i = 1; i < arr.length; i++) {
        int temp = arr[i];
        int j;
        for(j = i - 1; j >= 0 && arr[j] > temp; j--) {
            arr[j + 1] = arr[j];
        }
        arr[j + 1] = temp;
    }
}

5. 归并排序

const int maxn = 100;

void merge(int arr[], int l1, int r1, int l2, int r2) {
    int temp[maxn];
    int i = l1, j = l2, index = 0;
    while(i <= r1 && j <= r2) {
        if(arr[i] <= arr[j])
            temp[index++] = arr[i++];
        else
            temp[index++] = arr[j++];
    }
    while(i <= r1)
        temp[index++] = arr[i++];
    while(j <= r2)
        temp[index++] = arr[j++];
    for(int i = 0; i < index; i++) {
        arr[l1 + i] = temp[i];
    }
}

void mergeSort(int arr[], int left, int right) {
    if(left < right) {
        int mid = left + (right - left) / 2;
        mergeSort(arr, left, mid);
        mergeSort(arr, mid + 1, right);
        merge(arr, left, mid, mid + 1, right);
    }
}

6. 二分查找

public static int find(int[] arr, int num) {
    int start = 0, end = arr.length - 1;
    while(start <= end) {
        int mid = (start + end) / 2;
        if(arr[mid] == num)
            return mid;
        else if(arr[mid] > num)
            end = mid - 1;
        else
            start = mid + 1;
    }
    return -1;
}

7. 求递增序列中的第一个大于等于x的元素的值

int lower_bound(int A[], int left, int right, int x) {
    int mid;
    while(left < right) {
        mid = left + (right - left) / 2;
        if(A[mid] >= x)
            right = mid;
        else
            left = mid + 1;
    }
    return left;
}

8. 求递增序列中的第一个大于x的元素的值

int upper_bound(int A[], int left, int right, int x) {
    int mid;
    while(left < right) {
        mid = left + (right - left) / 2;
        if(A[mid] > x)
            right = mid;
        else
            left = mid + 1;
    }
    return left;
}