[Tony] WEEK 15 Solutions

longest-palindromic-substring/TonyKim9401.java

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+// TC: O(n^2)
+// for loop & nested while loop => total O(n^2)
+// SC: O(1)
+// uses only constant space => O(1)
+class Solution {
+    public String longestPalindrome(String s) {
+        int right = 0;
+        int left = 0;
+
+        for (int i = 0; i < s.length(); i++) {
+            int even = pad(s, i, i);
+            int odd = pad(s, i, i+1);
+            int max = Math.max(even, odd);
+
+            if (max > (right - left)) {
+                right = i + (max + 1) / 2;
+                left = i - max / 2;
+            }
+        }
+
+        return s.substring(left, right+1);
+    }
+
+    private int pad(String s, int start, int end) {
+        while (start >= 0 && end < s.length() && s.charAt(start) == s.charAt(end)) {
+            start -= 1;
+            end += 1;
+        }
+        return start - end;
+    }
+}

rotate-image/TonyKim9401.java

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+// TC: O(n^2)
+// visit all elements with nested for-loop
+// SC: O(1)
+// constant space complexity
+class Solution {
+    public void rotate(int[][] matrix) {
+        int n = matrix.length;
+
+        for (int i = 0; i < (n + 1) / 2; i++) {
+            for (int j = 0; j < n / 2; j++) {
+                int temp = matrix[n-1-j][i];
+                matrix[n-1-j][i] = matrix[n-1-i][n-1-j];
+                matrix[n-1-i][n-1-j] = matrix[j][n-1-i];
+                matrix[j][n-1-i] = matrix[i][j];
+                matrix[i][j] = temp;
+            }
+        }
+    }
+}

subtree-of-another-tree/TonyKim9401.java

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+// TC: O(n * m)
+// need to check all nodes and subRoot nodes in the worst case
+// SC: O(h1 + h2)
+// the high of root = h1 + the high of subRoot h2 => O(h1 + h2)
+class Solution {
+    public boolean isSubtree(TreeNode root, TreeNode subRoot) {
+        if (root == null) return false;
+
+        if (sameCheck(root, subRoot)) return true;
+
+        return isSubtree(root.left, subRoot) ||
+                isSubtree(root.right, subRoot);
+    }
+
+    private boolean sameCheck(TreeNode root, TreeNode subRoot) {
+        if (root == null || subRoot == null) {
+            return root == null && subRoot == null;
+        }
+
+        if (root.val != subRoot.val) return false;
+
+        return sameCheck(root.left, subRoot.left) &&
+                sameCheck(root.right, subRoot.right);
+    }
+}

validate-binary-search-tree/TonyKim9401.java

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+// TC: O(n)
+// visit all nodes to compare
+// SC: O(n)
+// recursion requires O(n) SC in the worst case
+class Solution {
+    public boolean isValidBST(TreeNode root) {
+        return dfs(root, Long.MIN_VALUE, Long.MAX_VALUE);
+    }
+
+    private boolean dfs(TreeNode node, long min, long max) {
+        if (node == null) return true;
+
+        if (node.val <= min || node.val >= max) return false;
+        return dfs(node.left, min, node.val) &&
+                dfs(node.right, node.val, max);
+    }
+}