diff --git a/longest-substring-without-repeating-characters/njngwn.java b/longest-substring-without-repeating-characters/njngwn.java
new file mode 100644
index 000000000..d31b689ba
--- /dev/null
+++ b/longest-substring-without-repeating-characters/njngwn.java
@@ -0,0 +1,24 @@
+// Time Complexity: O(n), n: s.length()
+// Space Complexity: O(n), n: s.length()
+class Solution {
+    public int lengthOfLongestSubstring(String s) {
+        int begin = 0;
+        int end = 0;
+        int maxLength = 0;
+
+        Set<Character> letterSet = new HashSet<>();
+        while (end < s.length()) {
+            // if the letter is not in the hashset, then add it into hashset and move end pointer
+            if (!letterSet.contains(s.charAt(end))) {
+                letterSet.add(s.charAt(end));
+                ++end;
+                maxLength = Math.max(maxLength, end - begin); // update maxlength
+            } else { // if the letter is in the hashset, then remove it and move begin pointer
+                letterSet.remove(s.charAt(begin));
+                ++begin;
+            }
+        }
+
+        return maxLength;
+    }
+}
diff --git a/number-of-islands/njngwn.java b/number-of-islands/njngwn.java
new file mode 100644
index 000000000..b375a61bf
--- /dev/null
+++ b/number-of-islands/njngwn.java
@@ -0,0 +1,30 @@
+// Time Complexity: O(m*n), m: number of row, n: number of column
+// Space Complexity: O(m*n), m: number of row, n: number of column, because of call stack
+class Solution {
+    void findLand(char[][] grid, int row, int col) {
+        if (row < 0 || row > grid.length-1 || col < 0 || col > grid[0].length-1) return;
+        if (grid[row][col] == '0') return;
+
+        grid[row][col] = '0';   // make element '0'(water)
+
+        findLand(grid, row-1, col);
+        findLand(grid, row+1, col);
+        findLand(grid, row, col-1);
+        findLand(grid, row, col+1);
+    }
+
+    public int numIslands(char[][] grid) {
+        int num = 0;
+
+        for (int i = 0; i < grid.length; ++i) {
+            for (int j = 0; j < grid[0].length; ++j) {
+                if (grid[i][j] == '1') {
+                    findLand(grid, i, j);
+                    num++;
+                }
+            }
+        }
+
+        return num;
+    }
+}
diff --git a/set-matrix-zeroes/njngwn.java b/set-matrix-zeroes/njngwn.java
new file mode 100644
index 000000000..b57420763
--- /dev/null
+++ b/set-matrix-zeroes/njngwn.java
@@ -0,0 +1,28 @@
+// Time Complexity: O(m*n)
+// Space Complexity: O(m+n)
+class Solution {
+    public void setZeroes(int[][] matrix) {
+        ArrayList<Integer> rowList = new ArrayList<>();
+        ArrayList<Integer> colList = new ArrayList<>();
+
+        for (int m = 0; m < matrix.length; ++m) {
+            for (int n = 0; n < matrix[0].length; ++n) {
+                if (matrix[m][n] != 0) continue;
+                if (!rowList.contains(m)) rowList.add(m);
+                if (!colList.contains(n)) colList.add(n);
+            }
+        }
+
+        for (Integer row : rowList) {
+            for (int n = 0;  n < matrix[0].length; ++n) {
+                matrix[row][n] = 0;
+            }
+        }
+
+        for (Integer col : colList) {
+            for (int n = 0;  n < matrix.length; ++n) {
+                matrix[n][col] = 0;
+            }
+        }
+    }
+}
diff --git a/unique-paths/njngwn.java b/unique-paths/njngwn.java
new file mode 100644
index 000000000..096dc97ee
--- /dev/null
+++ b/unique-paths/njngwn.java
@@ -0,0 +1,19 @@
+// Time Complexity: O(m*n), m: number of row, n: number of column
+// Space Complexity: O(m*n), m: number of row, n: number of column
+class Solution {
+    public int uniquePaths(int m, int n) {
+        int[][] pathMap = new int[m][n];
+
+        for (int i = 0; i < m; ++i) {
+            for (int j = 0; j < n; ++j) {
+                if (i == 0 || j == 0) {
+                    pathMap[i][j] = 1;
+                } else {
+                    pathMap[i][j] = pathMap[i-1][j] + pathMap[i][j-1];
+                }
+            }
+        }
+
+        return pathMap[m-1][n-1];
+    }
+}