LeetCode | Single Number II

原题描述：https://oj.leetcode.com/problems/single-number-ii/

Given an array of integers, every element appears three times
except for one. Find that single one.

解题思路

  方法1: 通过一个HashMap来统计数字出现的次数，如果数字出现次数达到3，则将其移出HashMap，那么最终剩下的即为只出现过一次的那个数字；

  方法2: 将数字表示为32位二进制数的形式，那么每个数字就等价于：32位二进制中，某些位的值为1。将这些数字按位相加，每一位统计1出现的次数并对3取余，那么最终的结果即为只出现过一次的那个数字。

  方法1和方法2都只需要O(N)的时间复杂度，但是方法1的空间复杂度为O(N)，方法2的空间复杂度虽然为O(1)，但是需要开辟一个大小32的整型数组。那么，不需要额外的空间，是否可以实现呢？

  答案是肯定的：参考 http://www.cnblogs.com/daijinqiao/p/3352893.html

实现代码

/**
* LeetCode | Single Number
* @author wzystal
*/
public class Solution {
// Given an array of integers, every element appears three times except for one. Find that single one.
// 时间复杂度O(N),空间复杂度O(N)
public static int singleNumber1(int[] A) {
Map<Integer, Integer> map = new HashMap<Integer, Integer>();
int len = A.length;
for (int i = 0; i < len; i++) {
if (map.containsKey(A[i])) {
int count = map.get(A[i]);
if (count == 2) {
map.remove(A[i]);
} else {
map.put(A[i], ++count);
}
} else {
map.put(A[i], 1);
}
}
return map.keySet().iterator().next();
}

// Given an array of integers, every element appears three times except for one. Find that single one.
// 时间复杂度O(N),空间复杂度O(1),但是需要开辟一个大小为32的整型数组
public static int singleNumber2(int[] A) {
if (A.length == 0)
return 0;
int[] bits = new int[32];
int result = 0;
for (int i = 0; i < 32; i++) {
for (int j = 0; j < A.length; j++) {
if (((A[j] >> i) & 1) == 1) {
bits[i] = (bits[i] + 1) % 3;
}
}
result |= (bits[i] << i);
}
return result;
}

// Given an array of integers, every element appears three times except for one. Find that single one.
// 时间复杂度O(N),空间复杂度O(1)
public static int singleNumber3(int[] A) {
int len = A.length;
if (len == 0)
return 0;
int ones = 0, twos = 0, threes = 0;
for (int i = 0; i < len; i++) {
twos |= (ones & A[i]);
ones ^= A[i];
threes = ~(ones & twos);
ones &= threes;
twos &= threes;
}
return ones;
}
}