[hyunolike] WEEK 02 solutions

3sum/hyunolike.java

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+class Solution {
+    public List<List<Integer>> threeSum(int[] nums) {
+        List<List<Integer>> result = new ArrayList<>();
+
+        int n = nums.length;
+        if (n < 3) return result;
+
+        Arrays.sort(nums);
+
+        for(int i = 0; i < n - 2; i++) {
+            if(i > 0 && nums[i] == nums[i-1]) continue;
+
+            int left = i + 1;
+            int right = n - 1;
+
+            while(left < right) {
+                int sum = nums[i] + nums[left] + nums[right];
+
+                if(sum < 0){
+                    left++;
+                }
+                else if(sum > 0) {
+                    right --;
+                }
+                else {
+                                       // sum == 0 → 조합 하나 찾음
+                    result.add(Arrays.asList(nums[i], nums[left], nums[right]));
+
+                    // left, right 중복값 스킵
+                    int leftVal = nums[left];
+                    int rightVal = nums[right];
+
+                    while (left < right && nums[left] == leftVal) left++;
+                    while (left < right && nums[right] == rightVal) right--;
+                }
+            }
+        }
+
+        return result;
+    }
+}

climbing-stairs/hyunolike.java

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+class Solution {
+    public int climbStairs(int n) {
+
+        // 경우의 수 하기 >> 피보나치?
+        // ways(n) = ways(n-1) + ways(n-2)
+
+        if(n == 1) return 1;
+        if(n == 2) return 2;
+
+        int[] dp = new int[n+1];
+        dp[1] = 1;
+        dp[2] = 2;
+
+        for(int i = 3; i <= n; i++) {
+            dp[i] = dp[i-1] + dp[i-2];
+        }
+
+        return dp[n];
+    }
+}

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

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+class Solution {
+    public int[] productExceptSelf(int[] nums) {
+
+        int n = nums.length;
+        int[] ans = new int[n];
+
+        // 1. 왼족 곱
+        int left = 1;
+        for(int i = 0; i < n; i++) {
+            ans[i] = left;
+            left *= nums[i];
+        }
+
+        // 2. 오른쪽 곱
+        int right = 1;
+        for(int i = n-1; i >=0; i--){
+            ans[i] *= right;
+            right *= nums[i];
+        }
+
+        return ans;
+    }
+}

valid-anagram/hyunolike.java

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+class Solution {
+    public boolean isAnagram(String s, String t) {
+        // 각 문자열 배열 char[] 로 변환
+        // 그리고 비교 (?)
+
+        char[] sArray = s.toCharArray();
+        char[] tArray = t.toCharArray();
+
+        Arrays.sort(sArray);
+        Arrays.sort(tArray);
+
+        return Arrays.equals(sArray, tArray);
+    }
+}

validate-binary-search-tree/hyunolike.java

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+/**
+ * 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;
+ *     }
+ * }
+ */
+class Solution {
+    public boolean isValidBST(TreeNode root) {
+        return isValid(root, null, null);     
+    }
+
+    private boolean isValid(TreeNode node, Integer min, Integer max) {
+        if (node == null) {
+            return true;
+        }
+
+        if (min != null && node.val <= min) return false;
+        if (max != null && node.val >= max) return false;
+
+        return isValid(node.left, min, node.val) && isValid(node.right, node.val, max);
+    }
+}