[TONY] WEEK 07 Solutions

longest-substring-without-repeating-characters/TonyKim9401.java

@@ -0,0 +1,25 @@
+// TC: O(n^2)
+// -> all elements can be retrived multiple times in the worst case
+// SC: O(1)
+// -> since declare, no more increase or decrease
+class Solution {
+    public int lengthOfLongestSubstring(String s) {
+        int max = 0;
+        int count = 0;
+        boolean[] checkList = new boolean[128];
+
+        for (int i = 0; i < s.length(); i++) {
+            int idx = s.charAt(i);
+            if (checkList[idx]) {
+                max = Math.max(max, count);
+                i -= count;
+                count = 0;
+                checkList = new boolean[128];
+            } else {
+                count += 1;
+                checkList[idx] = true;
+            }
+        }
+        return max = Math.max(max, count);
+    }
+}

number-of-islands/TonyKim9401.java

@@ -0,0 +1,28 @@
+// TC: O(n * m)
+// retrieve all elemetns, grid.length * grid[0].length
+// SC: O(n * m(
+// need to change all elements from 1 to 0 in the worst case
+class Solution {
+    int output = 0;
+    public int numIslands(char[][] grid) {
+        for (int i = 0; i < grid.length; i++) {
+            for (int j = 0; j < grid[0].length; j++) {
+                if (grid[i][j] == '1') {
+                    output += 1;
+                    countIslands(i, j, grid);
+                }
+            }
+        }
+        return output;
+    }
+
+    private void countIslands(int i, int j, char[][] grid) {
+        if (i < 0 || i >= grid.length || j < 0 || j >= grid[0].length) return;
+        if (grid[i][j] == '0') return;
+        grid[i][j] = '0';
+        countIslands(i+1, j, grid);
+        countIslands(i-1, j, grid);
+        countIslands(i, j+1, grid);
+        countIslands(i, j-1, grid);
+    }
+}

reverse-linked-list/TonyKim9401.java

@@ -0,0 +1,16 @@
+// TC: O(n)
+// -> visit all elements of head
+// SC: O(1)
+// -> constant space complexity
+class Solution {
+    public ListNode reverseList(ListNode head) {
+        ListNode node = null;
+        while (head != null) {
+            ListNode temp = head.next;
+            head.next = node;
+            node = head;
+            head = temp;
+        }
+        return node;
+    }
+}

set-matrix-zeroes/TonyKim9401.java

@@ -0,0 +1,32 @@
+// TC: O(n^2)
+// SC: O(1)
+class Solution {
+    public void setZeroes(int[][] matrix) {
+        boolean firstRow = false, firstCol = false;
+
+        for (int i = 0; i < matrix.length; i++) {
+            for (int j = 0; j < matrix[0].length; j++) {
+                if (matrix[i][j] == 0) {
+                    if (i == 0) firstRow = true;
+                    if (j == 0) firstCol = true;
+                    matrix[0][j] = 0;
+                    matrix[i][0] = 0;
+                }
+            }
+        }
+
+        for (int i = 1; i < matrix.length; i++) {
+            for (int j = 1; j < matrix[0].length; j++) {
+                if (matrix[i][0] == 0 || matrix[0][j] == 0) matrix[i][j] = 0;
+            }
+        }
+
+        if (firstRow) {
+            for (int j = 0; j < matrix[0].length; j++) matrix[0][j] = 0;
+        }
+
+        if (firstCol) {
+            for (int i = 0; i < matrix.length; i++) matrix[i][0] = 0;
+        }
+    }
+}

unique-paths/TonyKim9401.java

@@ -0,0 +1,18 @@
+// TC: O (m * n)
+// SC: O (m * n)
+// -> need to retrieve all elements
+class Solution {
+    public int uniquePaths(int m, int n) {
+        int[][] dp = new int[m][n];
+
+        for (int i = 0; i < m; i++) dp[i][0] = 1;
+        for (int j = 0; j < n; j++) dp[0][j] = 1;
+
+        for (int i = 1; i < m; i++) {
+            for (int j = 1; j < n; j++) {
+                dp[i][j] = dp[i - 1][j] + dp[i][j - 1];
+            }
+        }
+        return dp[m-1][n-1];
+    }
+}