Signed-off-by: yuduozhou <[email protected]>

Author: yuduozhou

UniqueBST.java

@@ -0,0 +1,12 @@
+public class Solution {
+    public int numTrees(int n) {
+        // Start typing your Java solution below
+        // DO NOT write main() function
+        if (n <= 1) return 1;
+        int result = 0;
+        for (int i = 1; i  <= n ; i++){
+            result += numTrees(i - 1) * numTrees(n - i);
+        }
+        return result;
+    }
+}

UniqueBST_II.java

@@ -0,0 +1,27 @@
+public class Solution {
+    public ArrayList<TreeNode> generateTrees(int n) {
+        // Start typing your Java solution below
+        // DO NOT write main() function
+        return getTrees(1, n);
+    }
+    public ArrayList<TreeNode> getTrees(int min, int max){
+        ArrayList<TreeNode> result = new ArrayList<TreeNode>();
+        if (min > max){
+            result.add(null);
+            return result;
+        }
+        for (int root = min; root <= max; root ++){
+            ArrayList<TreeNode> lefts = getTrees(min, root - 1);
+            ArrayList<TreeNode> rights = getTrees(root + 1, max);
+            for (int i = 0; i < lefts.size(); i++ ){
+                for (int j = 0; j < rights.size(); j ++){
+                    TreeNode rootNode = new TreeNode(root);
+                    rootNode.right = rights.get(j);
+                    rootNode.left = lefts.get(i);
+                    result.add(rootNode); 
+                }
+            }
+        }
+        return result;
+    }
+}

UniquePaths.java

@@ -0,0 +1,40 @@
+/* numeric theory */
+public class Solution {
+    public int uniquePaths(int m, int n) {
+        // Start typing your Java solution below
+        // DO NOT write main() function
+        if (-- m > -- n){
+            int temp = m;
+            m = n;
+            n = temp;
+        }
+        long denominator = 1, numerator = 1;
+        for (int i = 1; i <= m; i++)
+            denominator *= i;
+        for (int i = m + n; i > n; i --)
+            numerator *= i;
+        return (int) (numerator / denominator);
+    }
+}
+
+/* DP */
+public class Solution {
+    public int uniquePaths(int m, int n) {
+        // Start typing your Java solution below
+        // DO NOT write main() function
+        int[][] table = new int[m][n];
+        table[0][0] = 1;
+        for (int i = 1; i < m; i ++){
+            table[i][0] = table[i - 1][0];
+        }
+        for (int j = 1; j < n; j ++){
+            table[0][j] = table[0][j - 1];
+        }
+        for (int i  = 1; i < m; i ++){
+            for (int j = 1; j < n; j ++){
+                table[i][j] = table[i][j - 1] + table[i - 1][j];
+            }
+        }
+        return table[m - 1][n - 1];
+    }
+}