[kimjunyoung90] WEEK 02 solutions

3sum/kimjunyoung90.java

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+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+
+public class kimjunyoung90 {
+    public List<List<Integer>> threeSum(int[] nums) {
+        List<List<Integer>> answers = new ArrayList<>();
+        //숫자들을 미리 정렬해서 3번째 단계에서 추가적인 정렬을 없게 만들자..
+        Arrays.sort(nums);
+
+        // 1. 요소가 중복되지 않는 3개의 숫자 조합을 찾음
+        for (int i = 0; i < nums.length - 2; i++) {
+
+            //i 값이 전 요소랑 같은 경우 탐색 건너 띄기
+            if (i > 0 && nums[i] == nums[i - 1]) continue;
+
+            //two pointer 적용
+            int left = i + 1;
+            int right = nums.length - 1;
+
+            while (left < right) {
+                int sum = nums[i] + nums[left] + nums[right];
+                if (sum == 0) {
+                    answers.add(Arrays.asList(nums[i], nums[left], nums[right]));
+
+                    //left 중복 체크
+                    while (left < right && nums[left] == nums[left + 1]) left++;
+
+                    //right 중복 체크
+                    while (left < right && nums[right] == nums[right - 1]) right--;
+
+                    left++;
+                    right--;
+                } else if (sum < 0) {
+                    left++;
+                } else {
+                    right--;
+                }
+            }
+        }
+
+        return answers;
+    }
+}

climbing-stairs/kimjunyoung90.java

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+public class kimjunyoung90 {
+    public int climbStairs(int n) {
+        if(n == 1) return 1;
+        if(n == 2) return 2;
+
+        int a = 1;
+        int b = 2;
+
+        for(int i = 3; i <= n; i++) {
+            int c = a + b;
+            a = b;
+            b = c;
+        }
+        return b;
+    }
+}

product-of-array-except-self/kimjunyoung90.java

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+public class kimjunyoung90 {
+    public int[] productExceptSelf(int[] nums) {
+        int[] answers = new int[nums.length];
+
+        //왼쪽 값 계산
+        answers[0] = 1;
+        for (int i = 1; i < nums.length; i++) {
+            answers[i] = answers[i - 1] * nums[i - 1];
+        }
+
+        //오른쪽 값 계산
+        int right = 1;
+        for (int i = nums.length - 1; i >= 0; i--) {
+            answers[i] *= right;
+            //다음 값 right 계산
+            right *= nums[i];
+        }
+        return answers;
+    }
+}

valid-anagram/kimjunyoung90.java

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+public class kimjunyoung90 {
+    public boolean isAnagram(String s, String t) {
+        if(s.length() != t.length()) return false;
+
+        int[] count = new int[26];
+
+        for (int i = 0; i < s.length(); i++) {
+            count[s.charAt(i) - 'a']++;
+            count[t.charAt(i) - 'a']--;
+        }
+
+        for(int c : count) {
+            if (c != 0) return false;
+        }
+
+        return true;
+    }
+}

validate-binary-search-tree/kimjunyoung90.java

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+public class kimjunyoung90 {
+    public boolean isValidBST(TreeNode root) {
+        return validate(root, Long.MIN_VALUE, Long.MAX_VALUE);
+    }
+
+    public boolean validate(TreeNode node, long min, long max) {
+        if(node == null) return true;
+        if(node.val <= min || node.val >= max) return false;
+        return validate(node.left, min, node.val) && validate(node.right, node.val, max);
+    }
+}