41 - First Missing Positive

 JAVA

• class Solution {
      public int firstMissingPositive(int[] nums) {
          int n = nums.length;
          for (int i = 0; i < n; ++i) {
              while (nums[i] >= 1 && nums[i] <= n && nums[i] != nums[nums[i] - 1]) {
                  swap(nums, i, nums[i] - 1);
              }
          }
          for (int i = 0; i < n; ++i) {
              if (i + 1 != nums[i]) {
                  return i + 1;
              }
          }
          return n + 1;
      }
  
      private void swap(int[] nums, int i, int j) {
          int t = nums[i];
          nums[i] = nums[j];
          nums[j] = t;
      }
  }

 

C++

• class Solution {
  public:
      int firstMissingPositive(vector<int>& nums) {
          int n = nums.size();
          for (int i = 0; i < n; ++i) {
              while (nums[i] >= 1 && nums[i] <= n && nums[i] != nums[nums[i] - 1]) {
                  swap(nums[i], nums[nums[i] - 1]);
              }
          }
          for (int i = 0; i < n; ++i) {
              if (i + 1 != nums[i]) {
                  return i + 1;
              }
          }
          return n + 1;
      }
  };