Tip1(easy)

https://leetcode.com/problems/jewels-and-stones/

class Solution {
    public int numJewelsInStones(String J, String S) {
       int count=0;
        for(int i=0;i<S.length();i++)
        {
            for(int j=0;j<J.length();j++)
            {
                if(S.charAt(i)==J.charAt(j))
                    count++;
            }
        }
        return count;
    }
}

String -> char array 변환 : String::toCharArray(), Character type for loop

class Solution {
    public int numJewelsInStones(String J, String S) {
        Set<Character> jSet = new HashSet<>();
        for(int i=0; i < J.length(); i++) {
            jSet.add(J.charAt(i));
        }
        
        int ans = 0;
        for(int i=0; i < S.length(); i++) {
            if(jSet.contains(S.charAt(i))) {
                ans++;
            }
        }
        return ans;
    }
}

 

https://leetcode.com/problems/how-many-numbers-are-smaller-than-the-current-number/

class Solution {
    public int[] smallerNumbersThanCurrent(int[] nums) {
        int[] sort = new int[nums.length];
		System.arraycopy(nums, 0, sort, 0, nums.length);
		Arrays.sort(sort);
		Map<Integer, Integer> sortMap = new HashMap<>();
		int rank = 0;
		for(int i : sort) {
			sortMap.putIfAbsent(i, rank++);
		}

		int[] ans = new int[nums.length];
		for(int i=0; i<nums.length; i++) {
			ans[i] = sortMap.get(nums[i]);
		}
        return ans;
    }
}

System::arraycopy / 대신에 int[] . clone() 이 더 쉬움

Arrays.sort

Map::putIfAbsent

 

https://leetcode.com/problems/decompress-run-length-encoded-list/submissions/

class Solution {
    public int[] decompressRLElist(int[] nums) {
        List<Integer> ans = new ArrayList<>();
		for(int i=0; i<nums.length;i+=2) {
			int freq = nums[i];
			int val = nums[i+1];
			for(int j=0; j<freq; j++) {
				ans.add(val);
			}
		}
		int[] ans2 = new int[ans.size()];
		for(int i=0; i<ans.size(); i++) {
			ans2[i] = ans.get(i);
		}
		return ans2;
    }
}

ArrayList<Integer> <-> int[] 변환이 매우 불편함... 가능하면 둘중 하나만 사용하도록 하는게 좋을듯

ArrayList::add 는 Arrays.fill();로 대체 가능

 

https://leetcode.com/problems/subtract-the-product-and-sum-of-digits-of-an-integer/

class Solution {
    public int subtractProductAndSum(int n) {
        String strn = Integer.toString(n);
        int mult = 1;
        int sum = 0;
        for(Character c : strn.toCharArray()) {
            mult *= (c - '0');
            sum += (c - '0');
        }
        return mult - sum;
    }
}

https://leetcode.com/problems/create-target-array-in-the-given-order/

class Solution {
    public int[] createTargetArray(int[] nums, int[] index) {
        int[] target = new int[nums.length];
		Arrays.fill(target, -1);
		for(int i=0; i<nums.length; i++) {
			if(nums[i] != -1) {
				if (index.length - 1 - index[i] >= 0)
					System.arraycopy(target, index[i], target, index[i] + 1, index.length - 1 - index[i]);
			}
			Arrays.fill(target, index[i], index[i] + 1, nums[i]);
		}
		return target;
    }
}

System::arraycopy의 좋은 예