first commit

.idea/.gitignore

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+# Default ignored files
+/workspace.xml

.idea/misc.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="ProjectRootManager" version="2" languageLevel="JDK_1_8" default="true" project-jdk-name="1.8.0_231" project-jdk-type="JavaSDK">
+    <output url="file://$PROJECT_DIR$/out" />
+  </component>
+</project>

.idea/modules.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="ProjectModuleManager">
+    <modules>
+      <module fileurl="file://$PROJECT_DIR$/leetCode.iml" filepath="$PROJECT_DIR$/leetCode.iml" />
+    </modules>
+  </component>
+</project>

.idea/vcs.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="VcsDirectoryMappings">
+    <mapping directory="$PROJECT_DIR$" vcs="Git" />
+  </component>
+</project>

leetCode.iml

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+<?xml version="1.0" encoding="UTF-8"?>
+<module type="JAVA_MODULE" version="4">
+  <component name="NewModuleRootManager" inherit-compiler-output="true">
+    <exclude-output />
+    <content url="file://$MODULE_DIR$">
+      <sourceFolder url="file://$MODULE_DIR$/src" isTestSource="false" />
+    </content>
+    <orderEntry type="inheritedJdk" />
+    <orderEntry type="sourceFolder" forTests="false" />
+  </component>
+</module>

src/GCDOfStrings.java

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+public class GCDOfStrings {
+    public String gcdOfStrings(String str1, String str2) {
+        if (!(str1 + str2).equals(str2 + str1)) return "";
+        return str1.substring(0, gcd(str1.length(), str2.length()));
+    }
+
+    public int gcd(int a, int b) {
+        return a % b == 0 ? b : gcd(b, a % b);
+    }
+
+    public static void main(String[] args) {
+        GCDOfStrings gcdOfStrings = new GCDOfStrings();
+        System.out.println(gcdOfStrings.gcdOfStrings("ABCABCABC", "ABCABC"));
+    }
+}

src/TwoSum.java

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+import java.util.Arrays;
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * 给定一个整数数组 nums 和一个目标值 target，请你在该数组中找出和为目标值的那 两个 整数，并返回他们的数组下标。
+ * 你可以假设每种输入只会对应一个答案。但是，你不能重复利用这个数组中同样的元素。
+ */
+public class TwoSum {
+    public int[] twoSum(int[] nums, int target) {
+        Map<Integer, Integer> map = new HashMap<>();
+        for (int i = 0; i < nums.length; i++) {
+            if (map.containsKey(target - nums[i])) {
+                return new int[]{map.get(target - nums[i]), i};
+            }
+            map.put(nums[i], i);
+        }
+        throw new RuntimeException("no answer");
+    }
+
+    public static void main(String[] args) {
+        TwoSum twoSum = new TwoSum();
+        System.out.println(Arrays.toString(twoSum.twoSum(new int[]{3, 2, 4}, 6)));
+    }
+}