[donghyeon95] Week3

combination-sum/donghyeon95.java

@@ -0,0 +1,60 @@
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Comparator;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.stream.Collectors;
+
+class Solution {
+	private HashMap<Integer, List<String>> dp = new HashMap<>();
+	private HashSet<Integer> set;
+
+	public List<List<Integer>> combinationSum(int[] candidates, int target) {
+		set = new HashSet<>(Arrays.stream(candidates).boxed().toList());
+		recurse(target);
+		// Convert dp entries back to List<List<Integer>> for return
+		return dp.getOrDefault(target, new ArrayList<>()).stream()
+			.map(str -> Arrays.stream(str.split(" "))
+				.map(Integer::valueOf)
+				.collect(Collectors.toList()))
+			.collect(Collectors.toList());
+	}
+
+	public void recurse(int target) {
+		if (dp.containsKey(target)) return;
+
+		List<String> combinations = new ArrayList<>();
+		for (int i = 1; i < target + 1; i++) {
+			if (set.contains(i)) {
+				int remaining = target - i;
+				recurse(remaining);
+				if (dp.containsKey(remaining)) {
+					for (String combination : dp.get(remaining)) {
+						List<Integer> newCombination = new ArrayList<>(Arrays.stream(combination.split(" "))
+							.map(Integer::valueOf)
+							.toList());
+						newCombination.add(i);
+						newCombination.sort(Comparator.reverseOrder());
+
+						String newCombinationStr = newCombination.stream()
+							.map(String::valueOf)
+							.collect(Collectors.joining(" "));
+						if (!combinations.contains(newCombinationStr)) {
+							combinations.add(newCombinationStr);
+						}
+					}
+				}
+			}
+		}
+		if (set.contains(target)) {
+			String singleCombination = String.valueOf(target);
+			if (!combinations.contains(singleCombination)) {
+				combinations.add(singleCombination);
+			}
+		}
+		dp.put(target, combinations);
+	}
+
+}
+

maximum-subarray/donghyeon95.java

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+import java.util.Arrays;
+
+class Solution {
+	public int maxSubArray(int[] nums) {
+		int max = Integer.MIN_VALUE;
+		int current = 0;
+
+		for (int num: nums) {
+			System.out.println(num + " " +max);
+			if (current + num >=0) {
+				max = Math.max(max, current+num);
+				current = current+num;
+			} else {
+				current = 0;
+			}
+		}
+
+		// 전부 음수일 경우 => 가장 큰수 return
+		return max>=0? max:  Arrays.stream(nums).max().getAsInt();
+	}
+}
+

product-of-array-except-self/donghyeon95.java

@@ -0,0 +1,31 @@
+class Solution {
+	public int[] productExceptSelf(int[] nums) {
+		int[] result = new int[nums.length];
+		int[] right = new int[nums.length];
+		int[] left = new int[nums.length];
+
+		// -> 이쪽 방향으로 한번 계산
+		right[0] = nums[0];
+		for (int i=1; i<nums.length; i++) {
+			right[i] = right[i-1]*nums[i];
+		}
+
+		// <- 이쪽 방향으로 한번 계산
+		left[nums.length-1] = nums[nums.length-1];
+		for (int i=nums.length-2; i>-1; i--) {
+			left[i] = left[i+1]*nums[i];
+		}
+
+		// f(i) = right(i-1) * left(i+1)
+		result[0] = left[1];
+		result[nums.length-1] = right[nums.length-2];
+		for (int i=1; i<nums.length-1; i++) {
+			result[i] = right[i-1] * left[i+1];
+		}
+
+		return result;
+	}
+
+	// 2 포인터를 활용하면 O(1)가능
+}
+

reverse-bits/donghyeon95.java

@@ -0,0 +1,16 @@
+public class Solution {
+	// you need treat n as an unsigned value
+	public int reverseBits(int n) {
+		int result = 0;
+
+		for (int i = 0; i < 32; i++) {
+			// 왼쪽으로 비트를 한 칸 이동하고, n의 마지막 비트를 추가
+			result = (result << 1) | (n & 1);
+			// n을 오른쪽으로 한 칸 이동
+			n >>= 1;
+		}
+
+		return result;
+	}
+}
+

two-sum/donghyeon95.java

@@ -0,0 +1,31 @@
+import java.util.HashMap;
+import java.util.HashSet;
+
+class Solution {
+	public int[] twoSum(int[] nums, int target) {
+		// // O(N^2)
+		// for (int i =0; i< nums.length-1; i++) {
+		// 	for (int j=i+1;  j<nums.length; j++) {
+		// 		if (nums[i] + nums[j] == target)
+		// 			return new int[] {i, j};
+		// 	}
+		// }
+		// return null;
+
+		// O(N)
+		// HashMap 사용
+		HashMap<Integer, Integer> map = new HashMap<>();
+		for (int i=0; i<nums.length; i++) {
+			map.putIfAbsent(nums[i], i);
+		}
+
+		for (int i=0; i<nums.length; i++) {
+			int num = nums[i];
+			int anoterNum = target - num;
+			if (map.containsKey(anoterNum) && i!=map.get(anoterNum))
+				return new int[] {i, map.get(anoterNum)};
+		}
+		return null;
+	}
+}
+