diff --git a/container-with-most-water/njngwn.java b/container-with-most-water/njngwn.java
new file mode 100644
index 000000000..579e8073d
--- /dev/null
+++ b/container-with-most-water/njngwn.java
@@ -0,0 +1,28 @@
+// Time Complexity: O(n), n: height.length
+// Space Complexity: O(1)
+class Solution {
+    public int maxArea(int[] height) {
+        int maxWaterAmount = 0;
+        int leftLineIdx = 0;
+        int rightLineIdx = height.length-1;
+
+        while (leftLineIdx < rightLineIdx) {
+            int leftHeight = height[leftLineIdx];
+            int rightHeight = height[rightLineIdx];
+            int tempAmount = 0;
+
+            if (leftHeight < rightHeight) {
+                tempAmount = leftHeight * (rightLineIdx - leftLineIdx);
+                leftLineIdx++;
+            } else {
+                tempAmount = rightHeight * (rightLineIdx - leftLineIdx);
+                rightLineIdx--;
+            }
+
+            // update maximum amount
+            maxWaterAmount = tempAmount > maxWaterAmount ? tempAmount : maxWaterAmount;
+        }
+
+        return maxWaterAmount;
+    }
+}
diff --git a/design-add-and-search-words-data-structure/njngwn.java b/design-add-and-search-words-data-structure/njngwn.java
new file mode 100644
index 000000000..f636fd40b
--- /dev/null
+++ b/design-add-and-search-words-data-structure/njngwn.java
@@ -0,0 +1,63 @@
+class WordDictionary {
+    private static class CharDictionary {
+        HashMap<Character, CharDictionary> charMap;
+        boolean isEnd;
+
+        private CharDictionary() {
+            this.charMap = new HashMap<>();
+            this.isEnd = false;
+        }
+    }
+
+    private CharDictionary rootNode;
+
+    public WordDictionary() {
+        this.rootNode = new CharDictionary();
+    }
+
+    public void addWord(String word) {
+        CharDictionary currentNode = this.rootNode;
+
+        for (char ch : word.toCharArray()) {
+            if (!currentNode.charMap.containsKey(ch)) {
+                currentNode.charMap.put(ch, new CharDictionary());
+            }
+            currentNode = currentNode.charMap.get(ch);
+        }
+        currentNode.isEnd = true;
+    }
+
+    public boolean search(String word) {
+        return searchRecursive(word, this.rootNode, 0);
+    }
+
+    private boolean searchRecursive(String word, CharDictionary node, int index) {
+        // Base case
+        if (index == word.length()) {
+            return node.isEnd;
+        }
+
+        char ch = word.charAt(index);
+
+        if (ch == '.') {
+            for (CharDictionary childNode : node.charMap.values()) {
+                if (searchRecursive(word, childNode, index + 1)) {
+                    return true;
+                }
+            }
+            return false;
+        } else {
+            if (!node.charMap.containsKey(ch)) {
+                return false;
+            }
+            return searchRecursive(word, node.charMap.get(ch), index + 1);
+        }
+    }
+}
+
+/**
+ * Your WordDictionary object will be instantiated and called as such:
+ * WordDictionary obj = new WordDictionary();
+ * obj.addWord(word);
+ * boolean param_2 = obj.search(word);
+ */
diff --git a/longest-increasing-subsequence/njngwn.java b/longest-increasing-subsequence/njngwn.java
new file mode 100644
index 000000000..654a9d340
--- /dev/null
+++ b/longest-increasing-subsequence/njngwn.java
@@ -0,0 +1,23 @@
+// time complexity: O(nlogn), n: nums.length (logn because of binary search)
+// space complexity: O(n), n: nums.length
+class Solution {
+    public int lengthOfLIS(int[] nums) {
+        ArrayList<Integer> incSeqList = new ArrayList<Integer>();   // dp
+        incSeqList.add(nums[0]);
+
+        for (int num : nums) {
+            if (num > incSeqList.get(incSeqList.size()-1)) {
+                // add element to incSeqLit
+                incSeqList.add(num);
+            } else {
+                int idx = Collections.binarySearch(incSeqList, num);
+                if (idx < 0) {  // idx returns -(insertedPos + 1)
+                    int insertedIdx = -(idx + 1);
+                    incSeqList.set(insertedIdx, num);
+                }
+            }
+        }
+
+        return incSeqList.size();
+    }
+}
diff --git a/reverse-linked-list/njngwn.java b/reverse-linked-list/njngwn.java
new file mode 100644
index 000000000..94ee5ac9d
--- /dev/null
+++ b/reverse-linked-list/njngwn.java
@@ -0,0 +1,27 @@
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode() {}
+ *     ListNode(int val) { this.val = val; }
+ *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
+ * }
+ */
+// Time Complexity: O(n)
+// Sprace Compexity: O(1)
+class Solution {
+    public ListNode reverseList(ListNode head) {
+        ListNode prevNode = null;
+        ListNode currNode = head;
+
+        while (currNode != null) {
+            ListNode nextNode =  currNode.next;
+            currNode.next = prevNode;
+            prevNode = currNode;
+            currNode = nextNode;
+        }
+
+        return prevNode;
+    }
+}
diff --git a/spiral-matrix/njngwn.java b/spiral-matrix/njngwn.java
new file mode 100644
index 000000000..60190a138
--- /dev/null
+++ b/spiral-matrix/njngwn.java
@@ -0,0 +1,44 @@
+// Time Complexity: O(m+n), m: matrix.length, n: matrix[0].length
+// Space Complexity: O(m*n), m: matrix.length, n: matrix[0].length, because of arraylist for output
+class Solution {
+    public List<Integer> spiralOrder(int[][] matrix) {
+        int rowMin = 0;
+        int rowMax = matrix.length-1;
+        int colMin = 0;
+        int colMax = matrix[0].length-1;
+
+        ArrayList<Integer> orderedElements = new ArrayList<>();
+
+        while (rowMin <= rowMax && colMin <= colMax) {
+            // left to right
+            for (int col = colMin; col <= colMax; ++col) {
+                orderedElements.add(matrix[rowMin][col]);
+            }
+            rowMin++;
+
+            // top to bottom
+            for (int row = rowMin; row <= rowMax; ++row) {
+                orderedElements.add(matrix[row][colMax]);
+            }
+            colMax--;
+
+            // right to left
+            if (rowMin <= rowMax) {
+                for (int col = colMax; col >= colMin; --col) {
+                    orderedElements.add(matrix[rowMax][col]);
+                }
+            }
+            rowMax--;
+
+            // bottom to top
+            if (colMin <= colMax) {
+                for (int row = rowMax; row >= rowMin; --row) {
+                    orderedElements.add(matrix[row][colMin]);
+                }
+            }
+            colMin++;
+        }
+
+        return orderedElements;
+    }
+}
diff --git a/valid-parentheses/njngwn.java b/valid-parentheses/njngwn.java
new file mode 100644
index 000000000..3c18e535d
--- /dev/null
+++ b/valid-parentheses/njngwn.java
@@ -0,0 +1,24 @@
+// Time Complexity: O(n), n: s.length
+// Space Complexity: O(n), n: s.length (worst case: s="(((((((")
+class Solution {
+    public boolean isValid(String s) {
+        Stack<Character> bracketStack = new Stack<>();
+
+        for (char ch : s.toCharArray()) {
+            if (ch == '(' || ch == '{' || ch == '[') { // open bracket
+                bracketStack.push(ch);
+            } else {    // close bracket
+                if (bracketStack.empty()) {
+                    return false;
+                }
+
+                char sp = bracketStack.pop();
+                if (!((sp == '(' && ch == ')') || (sp == '{' && ch == '}') || (sp == '[' && ch == ']'))) {
+                    return false;
+                }
+            }
+        }
+
+        return bracketStack.empty();
+    }
+}