[sora0319] WEEK 02 solutions

3sum/sora0319.java

@@ -0,0 +1,37 @@
+import java.util.*;
+
+class Solution {
+    public List<List<Integer>> threeSum(int[] nums) {
+        List<List<Integer>> result = new ArrayList<>();
+        Arrays.sort(nums);
+        int n = nums.length;
+
+        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) {
+                    result.add(Arrays.asList(nums[i], nums[left], nums[right]));
+
+                    while (left < right && nums[left] == nums[left + 1]) left++;
+                    while (left < right && nums[right] == nums[right - 1]) right--;
+
+                    left++;
+                    right--;
+                } else if (sum < 0) {
+                    left++;
+                } else {
+                    right--;
+                }
+            }
+        }
+
+        return result;
+    }
+}
+

climbing-stairs/sora0319.java

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+class Solution {
+    public int climbStairs(int n) {
+        int[] stairs = new int[n+1];
+
+        stairs[0] = 1;
+        stairs[1] = 1;
+
+        for(int i = 2; i <= n; i++){
+            stairs[i] = stairs[i-1] + stairs[i-2];
+        }
+        return stairs[n];
+    }
+}
+

product-of-array-except-self/sora0319.java

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+class Solution {
+    public int[] productExceptSelf(int[] nums) {
+        int[] answer = new int[nums.length];
+        int multi = nums[0];
+        answer[0] = 1;
+
+        for(int i = 1; i < nums.length; i++){
+            answer[i] = multi;
+            multi *= nums[i];
+        }
+
+        multi = nums[nums.length-1];
+        for(int i = nums.length-2; i >= 0; i--){
+            answer[i] *= multi;
+            multi *= nums[i];
+        }
+
+        return answer;
+    }
+}
+

valid-anagram/sora0319.java

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

validate-binary-search-tree/sora0319.java

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