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算法2

    博客分类:
  • java
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A zero-indexed array A consisting of N integers is given. The dominator of array A is the value that occurs in more than half of the elements of A.

For example, consider array A such that

 

A[0] = 3    A[1] = 4    A[2] =  3
A[3] = 2    A[4] = 3    A[5] = -1
A[6] = 3    A[7] = 3

 

The dominator of A is 3 because it occurs in 5 out of 8 elements of A (namely in those with indices 0, 2, 4, 6 and 7) and 5 is more than a half of 8.

Write a function

class Solution { public int dominator(int[] A); }

that, given a zero-indexed array A consisting of N integers, returns index of any element of array A in which the dominator of A occurs. The function should return −1 if array A does not have a dominator.

Assume that:

  • N is an integer within the range [0..1,000,000];
  • each element of array A is an integer within the range [−2,147,483,648..2,147,483,647].

For example, given array A such that

 

A[0] = 3    A[1] = 4    A[2] =  3
A[3] = 2    A[4] = 3    A[5] = -1
A[6] = 3    A[7] = 3

 

the function may return 0, 2, 4, 6 or 7, as explained above.

Complexity:

  • expected worst-case time complexity is O(N);
  • expected worst-case space complexity is O(1), beyond input storage (not counting the storage required for input arguments).

Elements of input arrays can be modified.

 

 

 

解决方案:

时间复杂度和空间复杂度都能满足

 

public int dominator(int[] A) {
  int n = A.length;
  
  Hashtable<Integer, Integer> hash = new Hashtable<Integer, Integer>();
  for(int i = 0; i < n; i++) {
   int a = A[i];
   if(hash.containsKey(a)) {
    hash.put(a, (hash.get(a) + 1));
   } else {
    hash.put(a, 1);
   }
   if(hash.get(a) * 2 > n) {
    return i;
   }
  }

  return -1;
 }

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