Merge pull request #1232 from YoungSeok-Choi/feature/week-2

YoungSeok-Choi · web-flow · commit fc43e4c8c28b · 2025-04-14T10:32:19.000+09:00
[YoungSeok-Choi] Week 2 Solutions
diff --git a/3sum/YoungSeok-Choi.java b/3sum/YoungSeok-Choi.java
@@ -0,0 +1,65 @@
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Set;
+
+// NOTE: 실행시간 1.5초..
+// 시간 복잡도: O(n^2)
+class Solution {
+    public List<List<Integer>> threeSum(int[] nums) {
+
+        Arrays.sort(nums);
+        Set<List<Integer>> result = new HashSet<>();
+
+        for(int i = 0; i < nums.length - 2; i++) {
+            int startIdx = i + 1;
+            int endIdx = nums.length - 1;
+            while(startIdx < endIdx) {
+                int sum = nums[i] + nums[startIdx] + nums[endIdx];
+                if(sum == 0) {
+                    result.add(Arrays.asList(nums[i], nums[startIdx], nums[endIdx]));
+                }
+
+                if(sum > 0) {
+                    endIdx--;
+                } else {
+                    startIdx++;
+                }
+            }
+        }
+
+        return new ArrayList<>(result);   
+    }
+}
+
+
+
+//  309 / 314 testcases passed Time Limit Exceeded
+// NOTE: 시간복잡도 O(n^3)
+class WrongSolution {
+    public List<List<Integer>> threeSum(int[] nums) {
+        Set<List<Integer>> res = new HashSet<>();
+
+        List<Integer> temp = new ArrayList<>();
+
+        temp.remove(3);
+
+        for(int i = 0; i < nums.length; i++) {
+            for(int j = 0; j < i; j++) {
+                for(int k = 0; k < j; k++) {
+
+                    if((nums[i] + nums[j] + nums[k]) == 0) {
+                        List<Integer> solution = Arrays.asList(nums[i], nums[j], nums[k]);
+                        Collections.sort(solution);
+                        res.add(solution);
+                    }
+                }
+            }
+        }
+
+
+        return new ArrayList<>(res);
+    }
+}
diff --git a/climbing-stairs/YoungSeok-Choi.java b/climbing-stairs/YoungSeok-Choi.java
@@ -0,0 +1,21 @@
+// NOTE: 백준 N과 M문제와 거의 동일한 문제
+// memo(n) = memo(n-1)의 경우에 수에서 1칸을 올라가는 경우 + memo(n-2) 의 경우의 수에서 2 칸을 올라가는 경우의 점화식으로 표현됨.
+// 시간 복잡도: O(n)
+class Solution {
+    public int climbStairs(int n) {
+        int[] memo = new int[n + 1];
+
+
+        if(n < 3) {
+            return n;
+        }
+
+        memo[1] = 1;
+        memo[2] = 2;
+        for(int i = 3; i < memo.length; i++) {
+            memo[i] = memo[i - 1] + memo[i - 2];
+        }
+        
+        return memo[n];
+    }
+}
diff --git a/top-k-frequent-elements/YoungSeok-Choi.java b/top-k-frequent-elements/YoungSeok-Choi.java
@@ -0,0 +1,32 @@
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+
+// O(nlogn) 시간복잡도.
+
+class Solution {    
+    public int[] topKFrequent(int[] nums, int k) {
+        Map<Integer, Integer> freqMap = new HashMap<>();
+        int[] res = new int [k];
+
+        for(int i = 0; i < nums.length; i++) {
+            if(freqMap.containsKey(nums[i])) {
+                freqMap.put(nums[i], freqMap.get(nums[i]) + 1);
+            } else {
+                freqMap.put(nums[i], 1);
+            }
+        }
+
+        List<Map.Entry<Integer, Integer>> entList = new ArrayList<>(freqMap.entrySet());
+
+        entList.sort((a, b) -> b.getValue().compareTo(a.getValue()));
+        
+        for(int i = 0; i < res.length; i++) {
+            res[i] = entList.get(i).getKey();
+        }
+
+        return res;
+    }
+}
diff --git a/valid-anagram/YoungSeok-Choi.java b/valid-anagram/YoungSeok-Choi.java
@@ -0,0 +1,56 @@
+import java.util.HashMap;
+import java.util.Map;
+
+
+// NOTE: anagram 자체를 정확하게 이해하지 못해서 몇 번 틀렸던 문제.
+// 두 문자열의 길이가 다르면 anagram이 아니다.
+// 같은 알파뱃이 같은 개수가 있는지 확인을 하는게 문제의 핵심.
+
+// NOTE: 시간복잡도: O(n+m)
+class Solution {
+    public boolean isAnagram(String s, String t) {
+        Map<Character, Integer> sMap = new HashMap<>();
+        Map<Character, Integer> tMap = new HashMap<>();
+
+        int tLen = t.toCharArray().length;
+        int sLen = s.toCharArray().length;
+
+        if(tLen != sLen) {
+            return false;
+        }
+
+        for(char c: s.toCharArray()) {
+            if(sMap.containsKey(c)) {
+                int cnt = sMap.get(c);
+                sMap.put(c, cnt + 1);
+            } else {
+                sMap.put(c, 1);
+            }
+        }
+
+        for(char c: t.toCharArray()) {
+            if(tMap.containsKey(c)) {
+                int cnt = tMap.get(c);
+                tMap.put(c, cnt + 1);
+            } else {
+                tMap.put(c, 1);
+            }
+        }
+
+
+        for(Character c: sMap.keySet()) {
+            if(!tMap.containsKey(c)) {
+                return false;
+            }
+
+            int sMapCnt = sMap.get(c);
+            int tMapCnt = tMap.get(c);
+
+            if(sMapCnt != tMapCnt) {
+                return false;
+            }
+        }
+
+        return true;
+    }
+}
diff --git a/validate-binary-search-tree/YoungSeok-Choi.java b/validate-binary-search-tree/YoungSeok-Choi.java
@@ -0,0 +1,88 @@
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode() {}
+ *     TreeNode(int val) { this.val = val; }
+ *     TreeNode(int val, TreeNode left, TreeNode right) {
+ *         this.val = val;
+ *         this.left = left;
+ *         this.right = right;
+ *     }
+ * }
+ */
+// NOTE: 답지를 보고 풀었던 문제.. 정확하게 이해하는 과정이 필요하다.
+// TODO: 별도 블로그로 풀이과정을 정리할 것.
+class Solution {
+    public boolean isValidBST(TreeNode root) {
+        return dfs(root, Long.MIN_VALUE, Long.MAX_VALUE);
+    }
+
+    public boolean dfs(TreeNode cur, long min, long max) {
+        if(cur == null) return true;
+
+        if(cur.val <= min || cur.val >= max) return false;
+
+        return dfs(cur.left, min, cur.val) && dfs(cur.right, cur.val, max);
+    }          
+}
+
+class WrongSolution {
+    public boolean isValidBST(TreeNode root) {
+        return dfs(root, new ArrayList<>(Arrays.asList(root.val)));
+    }
+
+    public boolean dfs(TreeNode cur, List<Integer> path) {
+        
+        if(cur.left != null) {
+            int lVal = cur.left.val;
+            for(int i = 0; i < path.size(); i++) {
+                if(i == path.size() - 1) {
+
+                    if(lVal >= path.get(i)) {
+                        return false;
+                    }
+                    
+                } else {
+                    if(lVal < path.get(i)) {
+                        return false;
+                    }
+                }
+            }
+            
+            path.add(cur.left.val);
+            if(!dfs(cur.left, path)) return false;
+            path.remove(Integer.valueOf(cur.left.val));
+        }
+
+        if(cur.right != null) {
+            int rVal = cur.right.val;
+            for(int i = 0; i < path.size(); i++) {
+                if(i == path.size() - 1) {
+
+                    if(rVal <= path.get(i)) {
+                        return false;
+                    }
+                    
+                    
+                } else {
+                    if(rVal > path.get(i)) {
+                        return false;
+                    }
+                }
+            }
+
+            path.add(cur.right.val);
+            if(!dfs(cur.right, path)) return false;
+            path.remove(Integer.valueOf(cur.right.val));
+        }
+
+        return true;
+    }   
+}
