Added task 494.

Author: javadev

src/main/java/g0401_0500/s0494_target_sum/Solution.java

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+package g0401_0500.s0494_target_sum;
+
+// #Medium #Top_100_Liked_Questions #Array #Dynamic_Programming #Backtracking
+
+public class Solution {
+    public int findTargetSumWays(int[] nums, int s) {
+        int sum = 0;
+        s = Math.abs(s);
+        for (int i = 0; i < nums.length; i++) {
+            sum += nums[i];
+        }
+        // Invalid s, just return 0
+        if (s > sum || (sum + s) % 2 != 0) {
+            return 0;
+        }
+
+        int dp[][] = new int[(sum + s) / 2 + 1][nums.length + 1];
+        dp[0][0] = 1;
+        // empty knapsack must be processed specially
+        for (int i = 0; i < nums.length; i++) {
+            if (nums[i] == 0) {
+                dp[0][i + 1] = dp[0][i] * 2;
+            } else {
+                dp[0][i + 1] = dp[0][i];
+            }
+        }
+        for (int i = 1; i < dp.length; i++) {
+            for (int j = 0; j < nums.length; j++) {
+                dp[i][j + 1] += dp[i][j];
+                if (nums[j] <= i) {
+                    dp[i][j + 1] += dp[i - nums[j]][j];
+                }
+            }
+        }
+        return dp[(sum + s) / 2][nums.length];
+    }
+}

src/main/java/g0401_0500/s0494_target_sum/readme.md

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+494\. Target Sum
+
+Medium
+
+You are given an integer array `nums` and an integer `target`.
+
+You want to build an **expression** out of nums by adding one of the symbols `'+'` and `'-'` before each integer in nums and then concatenate all the integers.
+
+*   For example, if `nums = [2, 1]`, you can add a `'+'` before `2` and a `'-'` before `1` and concatenate them to build the expression `"+2-1"`.
+
+Return the number of different **expressions** that you can build, which evaluates to `target`.
+
+**Example 1:**
+
+**Input:** nums = \[1,1,1,1,1\], target = 3
+
+**Output:** 5
+
+**Explanation:**
+
+    There are 5 ways to assign symbols to make the sum of nums be target 3.
+    -1 + 1 + 1 + 1 + 1 = 3
+    +1 - 1 + 1 + 1 + 1 = 3
+    +1 + 1 - 1 + 1 + 1 = 3
+    +1 + 1 + 1 - 1 + 1 = 3
+    +1 + 1 + 1 + 1 - 1 = 3 
+
+**Example 2:**
+
+**Input:** nums = \[1\], target = 1
+
+**Output:** 1 
+
+**Constraints:**
+
+*   `1 <= nums.length <= 20`
+*   `0 <= nums[i] <= 1000`
+*   `0 <= sum(nums[i]) <= 1000`
+*   `-1000 <= target <= 1000`

src/test/java/g0401_0500/s0494_target_sum/SolutionTest.java

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+package g0401_0500.s0494_target_sum;
+
+import static org.hamcrest.CoreMatchers.equalTo;
+import static org.hamcrest.MatcherAssert.assertThat;
+
+import org.junit.jupiter.api.Test;
+
+class SolutionTest {
+    @Test
+    void findTargetSumWays() {
+        assertThat(new Solution().findTargetSumWays(new int[] {1, 1, 1, 1, 1}, 3), equalTo(5));
+    }
+
+    @Test
+    void findTargetSumWays2() {
+        assertThat(new Solution().findTargetSumWays(new int[] {1}, 1), equalTo(1));
+    }
+}