217. Contains Duplicate

Given an array of integers, find if the array contains any duplicates.

Your function should return true if any value appears at least twice in the array, and it should return false if every element is distinct.

Example 1:

Input: [1,2,3,1]
Output: true

Example 2:

Input: [1,2,3,4]
Output: false

Example 3:

Input: [1,1,1,3,3,4,3,2,4,2]
Output: true

Solution:

// 两遍扫描, Time:O(n), Spcae:O(1)
class Solution {
 public:
  bool containsDuplicate(vector<int> &nums) {
    if (nums.size() == 1 || nums.size() == 0) return 0;
    int n = nums.size();
    for (int i = 0; i < n-1; i++) {
      for (int j = i + 1; j < n; j++) {
        if (nums[i] == nums[j]) return 1;
      }
    }
    return 0;
  }
};

// 先排序, 对排序后的的数组1开始的元素, 检查前一个元素是否相等。
// Time:O(n*logn), Space:O(1)
class Solution {
 public:
  bool containsDuplicate(vector<int> &nums) {
    if (nums.size() == 1 || nums.size() == 0) return 0;

    sort(nums.begin(), nums.end());

    for (int i = 1; i < nums.size(); i++) {
      if (nums[i] == nums[i-1]) return 1;
    }
    return 0;
  }
};