[TONY] WEEK 04 Solutions

longest-consecutive-sequence/TonyKim9401.java

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+// TC: O(n)
+// SC: O(n)
+class Solution {
+    public int longestConsecutive(int[] nums) {
+        int output = 0;
+        Set<Integer> set = new HashSet<>();
+
+        for (int num : nums) set.add(num);
+
+        for (int num : nums) {
+            int count = 1;
+            if (!set.contains(num - count)){
+                while (set.contains(num + count)) {
+                    count += 1;
+                }
+            }
+            output = Math.max(output, count);
+        }
+        return output;
+    }
+}

maximum-product-subarray/TonyKim9401.java

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+// TC: O(n)
+// SC: O(1)
+class Solution {
+    public int maxProduct(int[] nums) {
+        int currentMax = nums[0];
+        int currentMin = nums[0];
+        int maxProduct = nums[0];
+
+        for (int i = 1; i < nums.length; i++) {
+            if (nums[i] < 0) {
+                int temp = currentMax;
+                currentMax = currentMin;
+                currentMin = temp;
+            }
+
+            currentMax = Math.max(nums[i], currentMax * nums[i]);
+            currentMin = Math.min(nums[i], currentMin * nums[i]);
+
+            maxProduct = Math.max(maxProduct, currentMax);
+        }
+
+        return maxProduct;
+    }
+}

missing-number/TonyKim9401.java

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+// TC: O(n log n)
+// SC: O(n)
+class Solution {
+    public int missingNumber(int[] nums) {
+        Arrays.sort(nums);
+        int idx = 1;
+        int n = nums.length;
+        for (; idx < n; idx++) {
+            if (nums[idx] - 1 != nums[idx-1]) return nums[idx] - 1;
+        }
+        return nums[idx-1] == n ? 0 : n;
+    }
+}

valid-palindrome/TonyKim9401.java

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+// TC: O(n)
+// SC: O(n)
+class Solution {
+    public boolean isPalindrome(String s) {
+        String target = checkString(s);
+        return checkPalindrome(target);
+    }
+
+    private String checkString(String s) {
+        StringBuilder sb = new StringBuilder();
+        for (char c : s.toCharArray()) {
+            if (c >= 'a' && c <= 'z') {
+                sb.append(c);
+            }
+            if (c >= 'A' && c <= 'Z') {
+                c = (char)(c - 'A' + 'a');
+                sb.append(c);
+            }
+            if (c >= '0' && c <= '9') {
+                sb.append(c);
+            }
+        }
+        return sb.toString();
+    }
+
+    private Boolean checkPalindrome(String target) {
+        int start = 0;
+        int end = target.length() - 1;
+
+        while (start < end) {
+            if (target.charAt(start) != target.charAt(end)) return false;
+            start += 1;
+            end -= 1;
+        }
+        return true;
+    }
+}

word-search/TonyKim9401.java

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+// TC: O(n * m * 4^k);
+// -> The size of board: n * m
+// -> Check 4 directions by the given word's length: 4^k
+// SC: O(n * m + k)
+// -> boolean 2D array: n * M
+// -> recursive max k spaces
+class Solution {
+    public boolean exist(char[][] board, String word) {
+        // Mark visited path to do not go back.
+        boolean[][] visit = new boolean[board.length][board[0].length];
+
+        for (int i = 0; i < board.length; i++) {
+            for (int j = 0; j < board[0].length; j++) {
+                if (wordSearch(i, j, 0, word, board, visit)) return true;
+            }
+        }
+        return false;
+    }
+
+    private boolean wordSearch(int i, int j, int idx, String word, char[][] board, boolean[][] visit) {
+
+        // When idx checking reach to the end of the length of the word then, return true
+        if (idx == word.length()) return true;
+
+        // Check if i and j are inside of the range
+        if (i < 0 || i >= board.length || j < 0 || j >= board[0].length) return false;
+
+        // Check if the coordinate equals to the charactor value
+        if (board[i][j] != word.charAt(idx)) return false;
+        if (visit[i][j]) return false;
+
+        // Mark the coordinate as visited
+        visit[i][j] = true;
+
+        // If visited, the target is gonna be the next charactor
+        idx += 1;
+
+        // If any direction returns true then it is true
+        if (
+            wordSearch(i+1, j, idx, word, board, visit) ||
+            wordSearch(i-1, j, idx, word, board, visit) ||
+            wordSearch(i, j+1, idx, word, board, visit) ||
+            wordSearch(i, j-1, idx, word, board, visit)
+        ) return true;
+
+        // If visited wrong direction, turns it as false
+        visit[i][j] = false;
+
+        return false;
+    }
+}