Merge pull request #790 from mintheon/main

[mintheon] Week3

Author: SamTheKorean

combination-sum/mintheon.java

@@ -0,0 +1,38 @@
+import java.util.ArrayDeque;
+import java.util.ArrayList;
+import java.util.Deque;
+import java.util.List;
+
+class Solution {
+  /**
+   시간복잡도: O(2^n)
+   공간복잡도: O(n)
+   */
+  public List<List<Integer>> combinationSum(int[] candidates, int target) {
+    List<List<Integer>> answer = new ArrayList<>();
+    Deque<Integer> nums = new ArrayDeque<>();
+
+    backtracking(candidates, answer, nums, target, 0, 0);
+
+    return answer;
+  }
+
+  protected void backtracking(int[] candidates, List<List<Integer>> answer, Deque<Integer> nums, int target, int start, int total) {
+    if(total > target) {
+      return;
+    }
+
+    if (total == target) {
+      answer.add(new ArrayList<>(nums));
+      return;
+    }
+
+    for(int i = start; i < candidates.length; i++) {
+      int num = candidates[i];
+      nums.push(num);
+
+      backtracking(candidates, answer, nums, target, i, total + num);
+      nums.pop();
+    }
+  }
+}

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

@@ -0,0 +1,23 @@
+class Solution {
+  /**
+   공간복잡도: O(n)
+   시간복잡도: O(n)
+   */
+  public int[] productExceptSelf(int[] nums) {
+    int[] answer = new int[nums.length];
+
+    answer[0] = 1;
+    for(int i = 1; i < nums.length; i++) {
+      answer[i] = answer[i - 1] * nums[i - 1];
+    }
+
+    int value = 1;
+    for(int i = nums.length - 1; i >= 0; i--) {
+
+      answer[i] = answer[i] * value;
+      value *= nums[i];
+    }
+
+    return answer;
+  }
+}

reverse-bits/mintheon.java

@@ -0,0 +1,24 @@
+public class Solution {
+  /**
+   시간복잡도: O(1) -> 루프는 항상 32번 반복되기 때문
+   공간복잡도: O(1)
+   */
+
+  // you need treat n as an unsigned value
+  public int reverseBits(int n) {
+    int answer = 0;
+    int index = 31;
+
+    while(n != 0) {
+      // n&1 : 마지막 비트를 추출
+      // << : 0을 패딩처리 시켜서 상위 자리수로 올려버림
+      answer += (n & 1) << index;
+
+      // >>> : 부호 상관없이 오른쪽으로 비트 이동
+      n = n >>> 1;
+      index--;
+    }
+
+    return answer;
+  }
+}

two-sum/mintheon.java

@@ -0,0 +1,26 @@
+import java.util.HashMap;
+import java.util.Map;
+
+class Solution {
+  /**
+   공간복잡도: O(n)
+   시간복잡도: O(n)
+   */
+
+  public int[] twoSum(int[] nums, int target) {
+    Map<Integer, Integer> numMap = new HashMap<>();
+
+    for(int i = 0; i < nums.length; i++) {
+      numMap.put(nums[i], i);
+    }
+
+    for(int i = 0; i < nums.length; i++) {
+      int pairNum = target - nums[i];
+      if(numMap.containsKey(pairNum) && numMap.get(pairNum) != i) {
+        return new int[]{i, numMap.get(target - nums[i])};
+      }
+    }
+
+    return new int[2];
+  }
+}