27

我的任务是创建一个方法，该方法将打印在排序数组中找到值 x 的所有索引。

我知道，如果我们只是从 0 到 N（数组长度）扫描数组，它的运行时间会是 O(n) 最坏的情况。由于将传递给方法的数组将被排序，我假设我可以利用使用二进制搜索，因为这将是 O(log n)。但是，这仅适用于数组具有唯一值的情况。因为二进制搜索将在第一次“找到”特定值之后完成。我正在考虑进行二进制搜索以在已排序的数组中找到 x，然后检查该索引之前和之后的所有值，但是如果数组包含所有 x 值，它似乎不会好得多。

我想我要问的是，有没有更好的方法来找到比 O(n) 更好的排序数组中特定值的所有索引？

public void PrintIndicesForValue42(int[] sortedArrayOfInts)
{
    // search through the sortedArrayOfInts

    // print all indices where we find the number 42. 
}

例如： sortedArray = { 1, 13, 42, 42, 42, 77, 78 } 将打印：“42 was found at Indices: 2, 3, 4”

4

10 回答 10

40

你会得到 O(lg n) 的结果

public static void PrintIndicesForValue(int[] numbers, int target) {
    if (numbers == null)
        return;

    int low = 0, high = numbers.length - 1;
    // get the start index of target number
    int startIndex = -1;
    while (low <= high) {
        int mid = (high - low) / 2 + low;
        if (numbers[mid] > target) {
            high = mid - 1;
        } else if (numbers[mid] == target) {
            startIndex = mid;
            high = mid - 1;
        } else
            low = mid + 1;
    }

    // get the end index of target number
    int endIndex = -1;
    low = 0;
    high = numbers.length - 1;
    while (low <= high) {
        int mid = (high - low) / 2 + low;
        if (numbers[mid] > target) {
            high = mid - 1;
        } else if (numbers[mid] == target) {
            endIndex = mid;
            low = mid + 1;
        } else
            low = mid + 1;
    }

    if (startIndex != -1 && endIndex != -1){
        for(int i=0; i+startIndex<=endIndex;i++){
            if(i>0)
                System.out.print(',');
            System.out.print(i+startIndex);
        }
    }
}
于 2013-12-03T18:01:45.943 回答
23

好吧，如果您确实有一个排序数组，您可以进行二进制搜索，直到找到您要查找的索引之一，然后从那里，其余的应该很容易找到，因为它们都在每个旁边-其他。

一旦你找到了你的第一个实例，你就会找到它之前的所有实例，然后找到它之后的所有实例。

使用该方法，您应该大致得到O(lg(n)+k)，其中k是您正在搜索的值的出现次数。

编辑：

而且，不，您将永远无法在小于O(k)的时间 内访问所有k值。


第二次编辑：这样我就可以感觉到我实际上在贡献一些有用的东西：

除了搜索 X 的第一次和最后一次出现之外，您还可以对第一次出现进行二进制搜索，对最后一次出现进行二进制搜索。这将导致O(lg(n))总数。一旦你这样做了，你就会知道所有的索引之间也包含 X（假设它是排序的）

您可以通过搜索检查值是否等于x并检查左侧的值（或右侧，取决于您是在寻找第一次出现还是最后一次出现）是否等于x来做到这一点。

于 2012-11-02T14:52:19.413 回答
3 
public void PrintIndicesForValue42(int[] sortedArrayOfInts) {
    int index_occurrence_of_42 = left = right = binarySearch(sortedArrayOfInts, 42);
    while (left - 1 >= 0) {
        if (sortedArrayOfInts[left-1] == 42)
            left--;
    }
    while (right + 1 < sortedArrayOfInts.length) {
        if (sortedArrayOfInts[right+1] == 42)
            right++;
    }
    System.out.println("Indices are from: " + left + " to " + right);
}

这将在 O(log(n) + #occurrences) 中运行 阅读并理解代码。这很简单。

于 2012-11-02T15:46:45.547 回答
2

我想出了使用二进制搜索的解决方案，唯一的事情是如果找到匹配项，则在两侧进行二进制搜索。

public static void main(String[] args) {
    int a[] ={1,2,2,5,5,6,8,9,10};
    System.out.println(2+" IS AVAILABLE  AT = "+findDuplicateOfN(a, 0, a.length-1, 2));
    System.out.println(5+" IS AVAILABLE  AT = "+findDuplicateOfN(a, 0, a.length-1, 5));
    int a1[] ={2,2,2,2,2,2,2,2,2};
    System.out.println(2+" IS AVAILABLE  AT = "+findDuplicateOfN(a1, 0, a1.length-1, 2));

    int a2[] ={1,2,3,4,5,6,7,8,9};
    System.out.println(10+" IS AVAILABLE  AT = "+findDuplicateOfN(a2, 0, a2.length-1, 10));
}

public static String findDuplicateOfN(int[] a, int l, int h, int x){
    if(l>h){
        return "";
    }
    int m = (h-l)/2+l;
    if(a[m] == x){
        String matchedIndexs = ""+m;
        matchedIndexs = matchedIndexs+findDuplicateOfN(a, l, m-1, x);
        matchedIndexs = matchedIndexs+findDuplicateOfN(a, m+1, h, x);
        return matchedIndexs;
    }else if(a[m]>x){
        return findDuplicateOfN(a, l, m-1, x);
    }else{
        return findDuplicateOfN(a, m+1, h, x);
    }
}


2 IS AVAILABLE  AT = 12 
5 IS AVAILABLE  AT = 43 
2 IS AVAILABLE  AT = 410236578 
10 IS AVAILABLE  AT =

我认为这仍然提供了 O(logn) 复杂度的结果。

于 2017-12-07T05:20:00.790 回答
1 
Find_Key(int arr[], int size, int key){
int begin = 0;
int end = size - 1;
int mid = end / 2;
int res = INT_MIN;

while (begin != mid)
{
    if (arr[mid] < key)
        begin = mid;
    else
    {
        end = mid;
        if(arr[mid] == key)
            res = mid;
    }
    mid = (end + begin )/2;
}
return res;
}

假设整数数组是升序排列的；返回键出现的第一个索引或 INT_MIN。在 O(lg n) 中运行。

于 2014-02-06T00:44:47.010 回答
1

如果您不需要使用二进制搜索，Hashmap 可能会起作用。

创建一个 HashMap ，其中Key是值本身，然后 value 是一个索引数组，其中该值在数组中。循环遍历您的数组，为每个值更新 HashMap 中的每个数组。

每个值的索引查找时间约为 O(1)，创建地图本身的时间约为 O(n)。

于 2012-11-02T14:53:27.720 回答
1

下面是返回搜索键在给定排序数组中展开的范围的 java 代码：

public static int doBinarySearchRec(int[] array, int start, int end, int n) {
    if (start > end) {
        return -1;
    }
    int mid = start + (end - start) / 2;

    if (n == array[mid]) {
        return mid;
    } else if (n < array[mid]) {
        return doBinarySearchRec(array, start, mid - 1, n);
    } else {
        return doBinarySearchRec(array, mid + 1, end, n);
    }
}

/**
 * Given a sorted array with duplicates and a number, find the range in the
 * form of (startIndex, endIndex) of that number. For example,
 * 
 * find_range({0 2 3 3 3 10 10}, 3) should return (2,4). find_range({0 2 3 3
 * 3 10 10}, 6) should return (-1,-1). The array and the number of
 * duplicates can be large.
 * 
 */
public static int[] binarySearchArrayWithDup(int[] array, int n) {

    if (null == array) {
        return null;
    }
    int firstMatch = doBinarySearchRec(array, 0, array.length - 1, n);
    int[] resultArray = { -1, -1 };
    if (firstMatch == -1) {
        return resultArray;
    }
    int leftMost = firstMatch;
    int rightMost = firstMatch;

    for (int result = doBinarySearchRec(array, 0, leftMost - 1, n); result != -1;) {
        leftMost = result;
        result = doBinarySearchRec(array, 0, leftMost - 1, n);
    }

    for (int result = doBinarySearchRec(array, rightMost + 1, array.length - 1, n); result != -1;) {
        rightMost = result;
        result = doBinarySearchRec(array, rightMost + 1, array.length - 1, n);
    }

    resultArray[0] = leftMost;
    resultArray[1] = rightMost;

    return resultArray;
}
于 2014-11-02T18:54:56.657 回答
1

log(n) 对最左边目标和最右边目标的二进制搜索的另一个结果。这是用 C++ 编写的，但我认为它非常易读。

这个想法是我们总是在left = right + 1. 因此，要找到最左边的目标，如果我们可以移动right到小于目标的最右边的数字，那么左边将是最左边的目标。

对于最左边的目标：

int binary_search(vector<int>& nums, int target){
    int n = nums.size();
    int left = 0, right = n - 1;

    // carry right to the greatest number which is less than target.
    while(left <= right){
        int mid = (left + right) / 2;
        if(nums[mid] < target)
            left = mid + 1;
        else
            right = mid - 1;
    }
    // when we are here, right is at the index of greatest number
    // which is less than target and since left is at the next, 
    // it is at the first target's index
    return left;
}

对于最右边的目标，想法非常相似：

int binary_search(vector<int>& nums, int target){
    while(left <= right){
        int mid = (left + right) / 2;
        // carry left to the smallest number which is greater than target.
        if(nums[mid] <= target)
            left = mid + 1;
        else
            right = mid - 1;
    }
    // when we are here, left is at the index of smallest number
    // which is greater than target and since right is at the next, 
    // it is at the first target's index
    return right;
}
于 2017-10-26T08:52:54.663 回答
1

它正在使用修改后的二进制搜索。它将是 O(LogN)。空间复杂度为 O(1)。我们调用了 BinarySearchModified 两次。一个用于查找元素的开始索引，另一个用于查找元素的结束索引。

private static int BinarySearchModified(int[] input, double toSearch)
    {
        int start = 0;
        int end = input.Length - 1;

        while (start <= end)
        {
            int mid = start + (end - start)/2;
            if (toSearch < input[mid]) end = mid - 1;
            else start = mid + 1;
        }

        return start;
    }


    public static Result GetRange(int[] input, int toSearch)
    {
        if (input == null) return new Result(-1, -1);

        int low = BinarySearchModified(input, toSearch - 0.5);

        if ((low >= input.Length) || (input[low] != toSearch)) return new Result(-1, -1);

        int high = BinarySearchModified(input, toSearch + 0.5);

        return new Result(low, high - 1);
    } 

 public struct Result
    {
        public int LowIndex;
        public int HighIndex;

        public Result(int low, int high)
        {
            LowIndex = low;
            HighIndex = high;
        }
    }
于 2016-01-28T06:18:37.880 回答
0 
public void printCopies(int[] array)
{
    HashMap<Integer, Integer> memberMap = new HashMap<Integer, Integer>();
    for(int i = 0; i < array.size; i++)
       if(!memberMap.contains(array[i]))
           memberMap.put(array[i], 1);
       else
       {
           int temp = memberMap.get(array[i]); //get the number of occurances
           memberMap.put(array[i], ++temp); //increment his occurance
       }

    //check keys which occured more than once
    //dump them in a ArrayList
    //return this ArrayList
 }

或者，您可以将它们的索引放在数组列表中，然后将其放在地图中而不是计数中，而不是计算出现次数。

   HashMap<Integer, ArrayList<Integer>> 
   //the integer is the value, the arraylist a list of their indices

public void printCopies(int[] array)
{
    HashMap<Integer, ArrayList<Integer>> memberMap = new HashMap<Integer, ArrayList<Integer>>();
    for(int i = 0; i < array.size; i++)
       if(!memberMap.contains(array[i]))
       {
           ArrayList temp = new ArrayList();
           temp.add(i);
           memberMap.put(array[i], temp);
       }
       else
       {
           ArrayList temp = memberMap.get(array[i]); //get the lsit of indices
           temp.add(i);
           memberMap.put(array[i], temp); //update the index list
       }

    //check keys which return lists with length > 1
    //handle the result any way you want
 }

呵呵，我想这将不得不发布。

 int predefinedDuplicate = //value here;
 int index = Arrays.binarySearch(array, predefinedDuplicate);
 int leftIndex, rightIndex;
 //search left
 for(leftIndex = index; array[leftIndex] == array[index]; leftIndex--); //let it run thru it
 //leftIndex is now the first different element to the left of this duplicate number string
 for(rightIndex = index; array[rightIndex] == array[index]; rightIndex++); //let it run thru it

 //right index contains the first different element to the right of the string
 //you can arraycopy this [leftIndex+1, rightIndex-1] string or just print it
 for(int i = leftIndex+1; i<rightIndex; i++)
 System.out.println(array[i] + "\t");
于 2012-11-02T14:58:19.447 回答