算法2
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
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A[0] = 3 A[1] = 4 A[2] = 3A[3] = 2 A[4] = 3 A[5] = -1A[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
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A[0] = 3 A[1] = 4 A[2] = 3A[3] = 2 A[4] = 3 A[5] = -1A[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.
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解决方案：
时间复杂度和空间复杂度都能满足
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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;
?}