Leet Code [Array]

Question

1. Two Sum
Given an array of integers nums and an integer target, return 
indices of the two numbers such that they add up to target.

Input: nums = [2,7,11,15], target = 9
Output: [0,1]


# Method 1 (Brute Force Approach) 
class Solution {
    public int[] twoSum(int[] nums, int target) {

        int i ,j;
        int a[]  = new int[2];
        int n= nums.length;
        int sum =0;

        for (i=0;i<n;i++){
             for (j=i+1;j<n;j++){
             
             sum=nums[i]+nums[j];

             if(target==sum){
                 a[0]=i;
                 a[1]=j;
             }}


        }
        return a;
    }
}
 complex city o(n2)

=========Method 2 =========

Using HashMap o(n)

class Solution {
    public int[] twoSum(int[] nums, int target) {
        int[] a  = new int[2];
        HashMap<Integer,Integer> map = new HashMap<>();
      for (int i=0;i<nums.length;i++){
          int value = target - nums[i];
           if(map.containsKey(value)){
               a[0] = i;
               a[1] = map.get(value);
           }
           map.put(nums[i],i);

      }
      return a;

        
    }
}

26(Leetcode). Remove Duplicates from Sorted Array

Given an integer array nums sorted in non-decreasing order,
remove the duplicates in-place such that each unique element appears only once.
 The relative order of the elements should be kept the same. 
Then return the number of unique elements in nums.

Example 1:

Input: nums = [1,1,2]
Output: 2, nums = [1,2,_]
Explanation: Your function should return k = 2, with the first two elements of nums being 1 and 2 respectively.
It does not matter what you leave beyond the returned k (hence they are underscores).

## Wrong approch 
class Solution {
    public int removeDuplicates(int[] nums) {

        Set<Integer> set = new HashSet();

        for(int i=0;i<nums.length;i++){
            set.add(nums[i]);

        }
        return set.size();
        
    }
}


Method 1

class Solution {
    public int removeDuplicates(int[] nums) {

       int j=0;

        for(int i=0;i<nums.length;i++){
            if(nums[i]!=nums[j]){
                j++;
               nums[j]=nums[i];
            }

        }
        return j+1;
        
    }
}

27. Remove Element

Given an integer array nums and an integer val, 
remove all occurrences of val in nums in-place.
The order of the elements may be changed. 
Then return the number of elements in nums which are not equal to val.

Example 1:

Input: nums = [3,2,2,3], val = 3
Output: 2, nums = [2,2,_,_]
Explanation: Your function should return k = 2, with the first two elements of nums being 2.
It does not matter what you leave beyond the returned k (hence they are underscores).

class Solution {
    public int removeElement(int[] nums, int val) {

        int j,count=0;
        for(j=0;j<nums.length;j++){

            if(nums[j] !=val){
                nums[count] =nums[j];
                    count++;
}

        }
        return count;


        
    }
}

66. Plus One

You are given a large integer represented as an integer array digits, 
where each digits[i] is the ith digit of the integer.

Example 1:

Input: digits = [1,2,3]
Output: [1,2,4]
Explanation: The array represents the integer 123.
Incrementing by one gives 123 + 1 = 124.
Thus, the result should be [1,2,4].

Example 3:

Input: digits = [9]
Output: [1,0]
Explanation: The array represents the integer 9.
Incrementing by one gives 9 + 1 = 10.
Thus, the result should be [1,0].