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xutao · xutao · commit 100ca5c1d145 · 2020-06-29T00:41:12.000+08:00
diff --git a/Week03/17.java b/Week03/17.java
@@ -0,0 +1,50 @@
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+class Solution {
+
+    private static List<String> result = new ArrayList<String>();
+    static  Map<String, String> phone = new HashMap<String, String>() {{
+        put("2", "abc");
+        put("3", "def");
+        put("4", "ghi");
+        put("5", "jkl");
+        put("6", "mno");
+        put("7", "pqrs");
+        put("8", "tuv");
+        put("9", "wxyz");
+    }};
+
+    public static List<String> letterCombinations(String digits) {
+        rec(0, digits, "");
+        return result;
+
+    }
+
+    public static void rec(int level, String digits, String now) {
+
+        if (level == digits.length()) {
+            result.add(now);
+            return;
+        }
+
+        String input = phone.get(digits.charAt(level));
+
+        if (input != null) {
+            for (int i = 0; i < input.length(); i++) {
+                rec(level + 1, digits, now + input.charAt(i));
+            }
+        } else {
+            rec(level + 1, digits, now);
+        }
+
+    }
+
+    public static void main(String[] args) {
+        letterCombinations("23");
+        System.out.println(result);
+    }
+
+}
diff --git a/Week03/22.java b/Week03/22.java
@@ -0,0 +1,26 @@
+import java.util.ArrayList;
+import java.util.List;
+
+class Solution {
+
+    private List<String> list = new ArrayList();
+
+    public List<String> generateParenthesis(int n) {
+        rec(0, 0, n, "");
+        return list;
+    }
+
+    public void rec(int left, int right, int n, String s) {
+        if (left == n && right == n) {
+            list.add(s);
+        }
+        if (left > right) {
+            rec(left, right+1, n, s + ")");
+        }
+        if (left < n) {
+            rec(left+1, right, n, s + "(");
+        }
+    }
+
+
+}
diff --git a/Week03/50.java b/Week03/50.java
@@ -0,0 +1,21 @@
+class Solution {
+    public double myPow(double x, int n) {
+        long N=n;
+        if (N < 0) {
+            N = -N;
+            x = 1.0 / x;
+        }
+        return fastPow(x, N);
+
+
+    }
+
+    public double fastPow(double x, long n) {
+        if (n == 0)
+            return 1.0;
+        double half = fastPow(x, n / 2);
+        return n % 2 == 1 ? half * half * x : half * half;
+    }
+
+
+}
diff --git a/Week03/78.java b/Week03/78.java
@@ -0,0 +1,24 @@
+import java.util.ArrayList;
+import java.util.List;
+
+class Solution {
+
+
+    private List<List<Integer>> result = new ArrayList<List<Integer>>;
+
+    public List<List<Integer>> subsets(int[] nums) {
+        dfs(nums, new ArrayList(),nums.length, 0);
+        return result;
+    }
+
+    public void dfs(int[] nums, List<Integer> array, int length, int level) {
+        if (array.size() == length) {
+            result.add(array);
+            return;
+        }
+        dfs(nums, array, length, level + 1);
+        array.add(nums[level]);
+        dfs(nums, array, length, level + 1);
+        array.remove(length - 1);
+    }
+}
