[samcho0608] WEEK 02 solutions

3sum/samcho0608.java

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+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+
+// link: https://leetcode.com/problems/3sum/
+// difficulty: Medium
+class Solution {
+    // Problem:
+    // * return: all triplets of elements in nums such that the sum == 0 (indices must differ)
+    // Solution:
+    // * Time Complexity: O(N^2)
+    // * Space Complexity: O(1)
+    public List<List<Integer>> threeSum(int[] nums) {
+        // sort for simplicity
+        // Time Complexity: O(N log N)
+        Arrays.sort(nums);
+
+        List<List<Integer>> answers = new ArrayList<>();
+
+        // if there are only positive or only negative numbers, there exist no solution
+        if(nums[0] > 0 || nums[nums.length-1] < 0) return answers;
+
+
+
+        // Time Complexity: O(N^2)
+        // * for loop * inner while loop = O(N) * O(N) = O(N^2)
+
+        // nums[i]: first of the triplet
+        // * skip positive because the other two will also be positive
+        for(int i = 0; i < nums.length && nums[i] <= 0; i++) {
+            // skip if same first of the triplet met
+            if(i != 0 && nums[i] == nums[i-1]) continue;
+
+            int numI = nums[i];
+
+            int left = i + 1;
+            int right = nums.length - 1;
+
+            // Time Complexity: O(N)
+            while(left < right) {
+                int numLeft = nums[left];
+                int numRight = nums[right];
+
+                int sum = numI + numLeft + numRight;
+
+                if(sum == 0) {
+                    answers.add(Arrays.asList(numI, numLeft, numRight));
+
+                    // skip same lefts and rights two prevent duplicate
+                    // * must update both left and right
+                    //    * e.g. if only left is moved, newNumLeft = 0 - (numI + numRight) and that can only be numLeft that's already visited
+                    // * Time Complexity: O(1) because there is no actual operation other than skipping
+                    while(left < nums.length && nums[left] == numLeft) left++;
+
+                    while(right > left && nums[right] == numRight) right--;
+                } else if(sum > 0) {
+                    right--;
+                } else {
+                    left++;
+                }
+            }
+        }
+
+        return answers;
+    }
+}

climbing-stairs/samcho0608.java

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+class Solution {
+    // Problem:
+    // * can take 1 or 2 steps
+    // * return: how many distinct ways to climb to the top(n)
+    // Solution:
+    // * Time Complexity: O(N)
+    //   * due to memoization(DP)
+    // * Space Complexity: O(N)
+    public int climbStairs(int n) {
+        // memo[i] = distinct steps to reach ith step
+        int[] memo = new int[n + 1];
+        memo[0] = 1;
+        memo[1] = 1;
+
+        for(int i = 2; i < n+1; i++) {
+            memo[i] = memo[i-1] + memo[i-2];
+        }
+
+        return memo[n];
+    }
+}

product-of-array-except-self/samcho0608.java

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+// link: https://leetcode.com/problems/product-of-array-except-self/submissions/1831301674/
+// difficulty: Medium
+class Solution1 {
+    // Problem:
+    // * return: array where answer[i] = product of all elements except nums[i]
+    // Solution:
+    // * Time Complexity: O(N)
+    // * Space Complexity: O(N)
+    public int[] productExceptSelf(int[] nums) {
+        int n = nums.length;
+
+        // prefixProds[i] = prod of all elements upto i (exclusive)
+        int[] prefixProds = new int[n];
+        for(int i = 0; i < n; i++) {
+            if(i == 0) {
+                prefixProds[i] = 1;
+                continue;
+            }
+
+            prefixProds[i] = prefixProds[i-1] * nums[i-1];
+        }
+
+        // suffixProds[i] = prod of all elements from end to i (exclusive)
+        int[] suffixProds = new int[n];
+        for(int i = n - 1; i >= 0; i--) {
+            if(i == n - 1) {
+                suffixProds[i] = 1;
+                continue;
+            }
+
+            suffixProds[i] = suffixProds[i+1] * nums[i+1];
+        }
+
+        // multiply prefix and suffix prods to find answers
+        int[] answers = new int[n];
+        for(int i = 0; i< n;i++) {
+            answers[i] = prefixProds[i] * suffixProds[i];
+        }
+
+        return answers;
+    }
+}
+
+// uses only 1 array whereas solution 1 uses 3
+class Solution2 {
+    // Problem:
+    // * return: array where answer[i] = product of all elements except nums[i]
+    // Solution:
+    // * Time Complexity: O(N)
+    // * Space Complexity: O(N)
+    public int[] productExceptSelf(int[] nums) {
+        int n = nums.length;
+
+        // prefixProds[i] = prod of all elements upto i (exclusive)
+        int[] answers = new int[n];
+        for(int i = 0; i < n; i++) {
+            if(i == 0) {
+                answers[i] = 1;
+                continue;
+            }
+
+            answers[i] = answers[i-1] * nums[i-1];
+        }
+
+        // use a single variable for suffix product
+        int suffixProd = 1;
+        for(int i = n - 2; i >= 0; i--) {
+            suffixProd *= nums[i+1];
+            answers[i] *= suffixProd;
+        }
+
+        return answers;
+    }
+}
+

valid-anagram/samcho0608.java

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+import java.util.HashMap;
+
+// link: https://leetcode.com/problems/valid-anagram/
+// difficulty: Easy
+class Solution1 {
+    // Problem:
+    // * return: is t an anagram of s
+    public boolean isAnagram(String s, String t) {
+        if(s.length() != t.length()) return false;
+
+        int n = s.length();
+
+        // Space Complexity: O(N)
+        HashMap<Character, Integer> freqS = new HashMap<>();
+        HashMap<Character, Integer> freqT = new HashMap<>();
+
+        // Time Complexity: O(N)
+        for(int i = 0; i < n; i++) {
+            char charS = s.charAt(i);
+            freqS.put(charS, freqS.getOrDefault(charS, 0) + 1);
+
+            char charT = t.charAt(i);
+            freqT.put(charT, freqT.getOrDefault(charT, 0) + 1);
+        }
+
+        // Time Complexity: O(N)
+        for(var entryS: freqS.entrySet()) {
+            int cntT = freqT.getOrDefault(entryS.getKey(), 0);
+            if(cntT != entryS.getValue()) return false;
+        }
+
+        return true;
+    }
+}
+
+// Map 하나만 사용하는 개선된 방식
+class Solution2 {
+    // Problem:
+    // * return: is t an anagram of s
+    public boolean isAnagram(String s, String t) {
+        if(s.length() != t.length()) return false;
+
+        int n = s.length();
+
+        // Space Complexity: O(N)
+        HashMap<Character, Integer> freq = new HashMap<>();
+
+        // Time Complexity: O(N)
+        for(int i = 0; i < n; i++) {
+            char charS = s.charAt(i);
+            freq.put(charS, freq.getOrDefault(charS, 0) + 1);
+
+            char charT = t.charAt(i);
+            freq.put(charT, freq.getOrDefault(charT, 0) - 1);
+        }
+
+        // Time Complexity: O(N)
+        for(int count: freq.values()) {
+            if(count != 0) return false;
+        }
+
+        return true;
+    }
+}

validate-binary-search-tree/samcho0608.java

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+// link: https://leetcode.com/problems/validate-binary-search-tree/description/
+// difficulty: Medium
+/**
+ * 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 Solution1 {
+    // Problem:
+    // * return: check if tree is a valid BST
+    // * left subtree contains keys strictly less than node's key
+    // * right subtree contains keys strictly greater than node's key
+    // * recursive structure
+    // Solution:
+    // * Time Complexity: O(N)
+    // * Space Complexity(in terms of call stack):
+    //   * O(N) if skewed
+    //   * O(log N) if not skewed
+    public boolean isValidBST(TreeNode root) {
+        // checklist:
+        // * is subtree a valid bst
+        // * left subtree : is max value of subtree less than node
+        // * right subtree : is min value of subtree greater than node
+        return isValid(root, null, null);
+    }
+
+    private boolean isValid(TreeNode node, Integer min, Integer max) {
+        if(node == null) return true;
+
+        int val = node.val;
+
+        if(min != null && val <= min) return false;
+        if(max != null && val >= max) return false;
+
+        if(!isValid(node.left, min, val)) return false;
+        if(!isValid(node.right, val, max)) return false;
+
+        return true;
+    }
+}
+
+// in-order traversal approach
+// * reads from left-most to right (strictly increasing val order expected)
+class Solution2 {
+    private Integer prev;
+
+    // Solution:
+    // * Time Complexity: O(N)
+    // * Space Complexity(in terms of call stack):
+    //   * O(N) if skewed
+    //   * O(log N) if not skewed
+    public boolean isValidBST(TreeNode root) {
+        prev = null;
+        return inorder(root);
+    }
+
+    private boolean inorder(TreeNode node) {
+        // reached end without failure
+        if(node == null) return true;
+
+        // inorder so travel left first
+        if(!inorder(node.left)) return false;
+
+        // if node value is not greater than prev, it isn't a BST in in-order
+        if(prev != null && node.val <= prev) return false;
+        prev = node.val;
+
+        return inorder(node.right);
+    }
+}