add 755

fishercoder1534 · fishercoder1534 · commit d955d2b764f6 · 2017-12-30T23:35:42.000-08:00
diff --git a/README.md b/README.md
@@ -22,6 +22,7 @@ Your ideas/fixes/algorithms are more than welcome!
 
 |  #  |      Title     |   Solutions   | Time          | Space         | Video  | Difficulty  | Tag                   
 |-----|----------------|---------------|---------------|---------------|--------|-------------|-------------
+|755|[Reach a Number](https://leetcode.com/problems/reach-a-number/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_755.java) | O(n) | O(1) | |Medium| Math
 |750|[Number Of Corner Rectangles](https://leetcode.com/problems/number-of-corner-rectangles/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_750.java) | O((m*n)^2) | O(1) | |Medium|
 |748|[Shortest Completing Word](https://leetcode.com/problems/shortest-completing-word/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_748.java) | O(n) | O(1) | |Easy|
 |747|[Largest Number Greater Than Twice of Others](https://leetcode.com/problems/largest-number-greater-than-twice-of-others/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_747.java) | O(n) | O(1) | |Easy|
diff --git a/src/main/java/com/fishercoder/solutions/_755.java b/src/main/java/com/fishercoder/solutions/_755.java
@@ -0,0 +1,47 @@
+package com.fishercoder.solutions;
+
+/**
+ * 755. Reach a Number
+ *
+ * You are standing at position 0 on an infinite number line. There is a goal at position target.
+ * On each move, you can either go left or right. During the n-th move (starting from 1), you take n steps.
+ * Return the minimum number of steps required to reach the destination.
+
+ Example 1:
+ Input: target = 3
+ Output: 2
+ Explanation:
+ On the first move we step from 0 to 1.
+ On the second step we step from 1 to 3.
+
+ Example 2:
+ Input: target = 2
+ Output: 3
+ Explanation:
+ On the first move we step from 0 to 1.
+ On the second move we step  from 1 to -1.
+ On the third move we step from -1 to 2.
+
+ Note:
+ target will be a non-zero integer in the range [-10^9, 10^9].
+ */
+
+public class _755 {
+  public static class Solution1 {
+    /**Two case:
+     * 1. go to the right, and reach the goal exactly.
+     * 2. go over the goal by several steps:
+     *    by even number, then you can choose one of the steps that went right to go back to the left (the step is half of what you went over)
+     *    by odd number, then you keep going until you are over by an even number.*/
+    public int reachNumber(int target) {
+      int absTarget = Math.abs(target);
+      int steps = 1;
+      int sum = 0;
+      while (sum < absTarget || (sum - absTarget) % 2 == 1) {
+        sum += steps;
+        steps++;
+      }
+      return steps - 1;
+    }
+  }
+}
diff --git a/src/test/java/com/fishercoder/_755Test.java b/src/test/java/com/fishercoder/_755Test.java
@@ -0,0 +1,96 @@
+package com.fishercoder;
+
+import com.fishercoder.solutions._755;
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+
+public class _755Test {
+  private static _755.Solution1 solution1;
+
+  @BeforeClass
+  public static void setup() {
+    solution1 = new _755.Solution1();
+  }
+
+  @Test
+  public void test4() {
+    assertEquals(1, solution1.reachNumber(1));
+  }
+
+  @Test
+  public void test2() {
+    assertEquals(3, solution1.reachNumber(2));
+  }
+
+  @Test
+  public void test1() {
+    assertEquals(2, solution1.reachNumber(3));
+  }
+
+  @Test
+  public void test3() {
+    assertEquals(3, solution1.reachNumber(4));
+  }
+
+  @Test
+  public void test5() {
+    assertEquals(5, solution1.reachNumber(5));
+  }
+
+  @Test
+  public void test6() {
+    assertEquals(3, solution1.reachNumber(6));
+  }
+
+  @Test
+  public void test7() {
+    assertEquals(5, solution1.reachNumber(7));
+  }
+
+  @Test
+  public void test8() {
+    assertEquals(4, solution1.reachNumber(8));
+  }
+
+  @Test
+  public void test9() {
+    assertEquals(5, solution1.reachNumber(9));
+  }
+
+  @Test
+  public void test10() {
+    assertEquals(4, solution1.reachNumber(10));
+  }
+
+  @Test
+  public void test11() {
+    assertEquals(15, solution1.reachNumber(100));
+  }
+
+  @Test
+  public void test12() {
+    assertEquals(47, solution1.reachNumber(1000));
+  }
+
+  @Test
+  public void test13() {
+    assertEquals(143, solution1.reachNumber(10000));
+  }
+
+  @Test
+  public void test14() {
+    assertEquals(447, solution1.reachNumber(100000));
+  }
+
+  @Test
+  public void test15() {
+    assertEquals(1415, solution1.reachNumber(1000000));
+  }
+
+  @Test
+  public void test16() {
+    assertEquals(4472, solution1.reachNumber(10000000));
+  }
+}
