[iam-edwin] WEEK 02 solutions

3sum/iam-edwin.java

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+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+
+class Solution {
+    public List<List<Integer>> threeSum(int[] nums) {
+        List<List<Integer>> result = new ArrayList<>();
+
+        Map<Integer, Integer> counter = new HashMap<>();
+        for (int num : nums) {
+            int count = counter.getOrDefault(num, 0);
+            counter.put(num, count + 1);
+        }
+
+        Set<Integer> keySet = new HashSet<>(counter.keySet());
+        for (int num1 : keySet) {
+            int num1Count = counter.get(num1);
+            if (num1Count > 1) {
+                counter.put(num1, num1Count - 1);
+            } else {
+                counter.remove(num1);
+            }
+
+            for (int num2 : counter.keySet()) {
+                int num3 = -num1 - num2;
+                int count = counter.getOrDefault(num3, 0);
+                if (((num2 == num3 && count >= 2) || (num2 != num3 && count >= 1)) && num3 >= num2) {
+                    result.add(List.of(num1, num2, num3));
+                }
+            }
+
+            counter.remove(num1);
+        }
+
+        return result;
+    }
+}

climbing-stairs/iam-edwin.java

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

product-of-array-except-self/iam-edwin.java

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+class Solution {
+    public int[] productExceptSelf(int[] nums) {
+        int[] left = new int[nums.length + 1];
+        int[] right = new int[nums.length + 1];
+        left[0] = 1;
+        right[nums.length] = 1;
+
+        for (int i = 0; i < nums.length; i++) {
+            left[i + 1] = left[i] * nums[i];
+            right[nums.length - i - 1] = right[nums.length - i] * nums[nums.length - i - 1];
+        }
+
+        int[] result = new int[nums.length];
+        for (int i = 0; i < nums.length; i++) {
+            result[i] = left[i] * right[i + 1];
+        }
+        return result;
+    }
+
+    public int[] productExceptSelfV2(int[] nums) {
+        int[] result = new int[nums.length];
+
+        int curr = 1;
+        result[0] = curr;
+        for (int i = 1; i < nums.length; i++) {
+            curr *= nums[i - 1];
+            result[i] = curr;
+        }
+
+        curr = 1;
+        result[nums.length - 1] *= curr;
+        for (int i = nums.length - 2; i >= 0; i--) {
+            curr *= nums[i + 1];
+            result[i] *= curr;
+        }
+
+        return result;
+    }
+}

valid-anagram/iam-edwin.java

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+import java.util.HashMap;
+import java.util.Map;
+
+class Solution {
+    public boolean isAnagram(String s, String t) {
+        Map<Character, Integer> sMap = new HashMap<>();
+        for (char c : s.toCharArray()) {
+            Integer count = sMap.getOrDefault(c, 0);
+            sMap.put(c, count + 1);
+        }
+
+        Map<Character, Integer> tMap = new HashMap<>();
+        for (char c : t.toCharArray()) {
+            Integer count = tMap.getOrDefault(c, 0);
+            tMap.put(c, count + 1);
+        }
+
+        for (char key : sMap.keySet()) {
+            if (!sMap.get(key).equals(tMap.get(key))) {
+                return false;
+            }
+            tMap.remove(key);
+        }
+
+        return tMap.isEmpty();
+    }
+}

validate-binary-search-tree/iam-edwin.java

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+class Solution {
+    public boolean isValidBST(TreeNode root) {
+        return (root.left == null || isValidBSTMax(root.left, root.val))
+                && (root.right == null || isValidBSTMin(root.right, root.val));
+    }
+
+    private boolean isValidBSTMax(TreeNode root, int max) {
+        if (root.val >= max) {
+            return false;
+        }
+
+        return (root.left == null || isValidBSTMax(root.left, root.val))
+                && (root.right == null || isValidBSTMinMax(root.right, root.val, max));
+    }
+
+    private boolean isValidBSTMin(TreeNode root, int min) {
+        if (root.val <= min) {
+            return false;
+        }
+
+        return (root.left == null || isValidBSTMinMax(root.left, min, root.val))
+                && (root.right == null || isValidBSTMin(root.right, root.val));
+    }
+
+    private boolean isValidBSTMinMax(TreeNode root, int min, int max) {
+        if (root.val >= max || root.val <= min) {
+            return false;
+        }
+
+        return (root.left == null || isValidBSTMinMax(root.left, min, root.val))
+                && (root.right == null || isValidBSTMinMax(root.right, root.val, max));
+    }
+
+    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;
+        }
+    }
+}