solve problem Array Partition I

Author: P-ppc

Diff for: README.md

@@ -18,4 +18,5 @@ All solutions will be accepted!
 |617|[Merge Two Binary Trees](https://leetcode-cn.com/problems/merge-two-binary-trees/description/)|[java/py/js](./algorithms/MergeTwoBinaryTrees)|Easy|
 |226|[Invert Binary Tree](https://leetcode-cn.com/problems/invert-binary-tree/description/)|[java/py/js](./algorithms/InvertBinaryTree)|Easy|
 |693|[Binary Number With Alternating Bits](https://leetcode-cn.com/problems/binary-number-with-alternating-bits/description/)|[java/py/js](./algorithms/BinaryNumberWithAlternatingBits)|Easy|
-|108|[Convert Sorted Array To Binary Search Tree](https://leetcode-cn.com/problems/convert-sorted-array-to-binary-search-tree/description/)|[java/py/js](./algorithms/ConvertSortedArrayToBinarySearchTree)|Easy|
+|108|[Convert Sorted Array To Binary Search Tree](https://leetcode-cn.com/problems/convert-sorted-array-to-binary-search-tree/description/)|[java/py/js](./algorithms/ConvertSortedArrayToBinarySearchTree)|Easy|
+|561|[Array Partition I](https://leetcode-cn.com/problems/array-partition-i/description/)|[java/py/js](./algorithms/ArrayPartitionI)|Easy|

Diff for: algorithms/ArrayPartitionI/README.md

@@ -0,0 +1,2 @@
+# Array Partition I
+Sort the array, then sum the odd term

Diff for: algorithms/ArrayPartitionI/Solution.java

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+class Solution {
+    public int arrayPairSum(int[] nums) {
+        Arrays.sort(nums);
+        int sum = 0;
+        for (int i = 0; i < nums.length; i++) {
+            if (i % 2 == 0) {
+                sum += nums[i];
+            }
+        }
+        return sum;
+    }
+}

Diff for: algorithms/ArrayPartitionI/solution.js

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+/**
+ * @param {number[]} nums
+ * @return {number}
+ */
+var arrayPairSum = function(nums) {
+    return nums.sort((x, y) => y - x).filter((d, i) => i % 2 === 0).reduce((x, y) => x + y, 0)
+};

Diff for: algorithms/ArrayPartitionI/solution.py

@@ -0,0 +1,7 @@
+class Solution(object):
+    def arrayPairSum(self, nums):
+        """
+        :type nums: List[int]
+        :rtype: int
+        """
+        return sum(sorted(nums)[::2])