add: Target Sum

Author: hijiangtao

README.md

@@ -56,6 +56,7 @@ This is the solutions collection of my LeetCode submissions, most of them are pr
 |404|[Sum of Left Leaves](https://leetcode.com/problems/sum-of-left-leaves/) | [JavaScript](./src/sum-of-left-leaves/res.js)|Easy|
 |416|[Partition Equal Subset Sum](https://leetcode.com/problems/partition-equal-subset-sum/) | [JavaScript](./src/partition-equal-subset-sum/res.js)|Medium|
 |434|[Number of Segments in a String](https://leetcode.com/problems/number-of-segments-in-a-string/) | [JavaScript](./src/number-of-segments-in-a-string/res.js)|Easy|
+|494|[Target Sum](https://leetcode.com/problems/target-sum/) | [JavaScript](./src/target-sum/res.js)|Medium|
 |516|[Longest Palindromic Subsequence](https://leetcode.com/problems/longest-palindromic-subsequence/) | [JavaScript](./src/longest-palindromic-subsequence/res.js)|Medium|
 |523|[Continuous Subarray Sum](https://leetcode.com/problems/continuous-subarray-sum/) | [JavaScript](./src/continuous-subarray-sum/res.js)|Medium|
 ||[]() | [](./src//res.js)||

src/target-sum/res.js

@@ -0,0 +1,50 @@
+/**
+ * res.js
+ * @authors Joe Jiang ([email protected])
+ * @date    2017-04-12 21:55:49
+ * 
+ * You are given a list of non-negative integers, a1, a2, ..., an, and a target, S. Now you have 2 symbols + and -. For each integer, you should choose one from + and - as its new symbol.
+ * 
+ * Find out how many ways to assign symbols to make sum of integers equal to target S.
+ * 
+ * Solution idea: https://discuss.leetcode.com/topic/76243/java-15-ms-c-3-ms-o-ns-iterative-dp-solution-using-subset-sum-with-explanation
+ * 
+ * @param {number[]} nums
+ * @param {number} S
+ * @return {number}
+ */
+let findTargetSumWays = function(nums, S) {
+   let len = nums.length, 
+        res = 0;
+   
+   for (let i=0; i<len; i++) {
+       res += nums[i];
+   }
+   if ((res < S && res > -S) || (res+S) & 1) return 0;
+   
+   return count(nums, len, (res+S)>>>1);
+};
+
+/**
+ * 计算子集存在满足等于 S 的情况数
+ * @param  {[type]} nums 数组
+ * @param  {[type]} len  数组长度
+ * @param  {[type]} S    计算总和
+ * @return {[type]}      [description]
+ */
+let count = function(nums, len, S) {
+    let arr = new Array(S+1);
+    arr[0] = 1;
+    for (let i=1; i<=S; i++) {
+        arr[i] = 0;
+    }
+    
+    // 遍历 nums 更新 arr 数组
+    for (let i=0; i<len; i++) {
+        for (let j=S; j>=nums[i]; j--) {
+            arr[j] += arr[j-nums[i]];
+        }
+    }
+    
+    return arr[S];
+}