add DifferenceArray.java, update readme

Author: yennanliu

README.md

@@ -199,7 +199,7 @@
 ||Priority Queue (`unsorted`) | [Java](./algorithm/java/UnorderedMaxPQ.java) || | | AGAIN| 
 ||LRU cache | [Python](./algorithm/python/lru_cache.py) || LC 146 | | | AGAIN| 
 ||LFU Cache | [Python](./algorithm/python/lfu_cache.py) || LC 460 | | | AGAIN| 
-
+||DifferenceArray | [Java](./algorithm/java/DifferenceArray.java) || LC 1109, 370 | | | AGAIN| 
 
 ## Array
 

leetcode_java/src/main/java/AlgorithmJava/DifferenceArray.java

@@ -0,0 +1,45 @@
+package AlgorithmJava;
+
+// https://github.com/yennanliu/CS_basics/blob/master/doc/cheatsheet/difference_array.md
+
+import java.util.Arrays;
+
+public class DifferenceArray {
+
+  // attr
+  int[] array;
+
+  // method
+  public int[] getDifferenceArray(int[][] input, int n) {
+
+    /** LC 1109. Corporate Flight Bookings input : [start, end, seats]
+     *
+     *  NOTE !!!
+     *
+     *   in java, index start from 0;
+     *   but in LC 1109, index start from 1
+     *
+     */
+    int[] tmp = new int[n + 1];
+    for (int[] x : input) {
+      int start = x[0];
+      int end = x[1];
+      int seats = x[2];
+
+      // add
+      tmp[start] += seats;
+
+      // subtract
+      if (end + 1 <= n) {
+        tmp[end + 1] -= seats;
+      }
+    }
+
+    for (int i = 1; i < tmp.length; i++) {
+      //tmp[i] = tmp[i - 1] + tmp[i];
+        tmp[i] += tmp[i - 1];
+    }
+
+    return Arrays.copyOfRange(tmp, 1, n+1);
+  }
+}