[Leetcode 494]目标和 Target Sum

 

【题目】

若干个数字间，使用加法或减法连接，最终得到目标和target，求所有满足条件解的个数

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

 

【思路】

本体只有加法或减法，数字不可拆（比如120就是120，不能拆成1、20）

即每个数字都有加减两种可能，遍历所有情况即可

*关联题282 难

【代码】

 

class Solution {

    int count = 0;
    public int findTargetSumWays(int[] nums, int target) {
        calculate(nums, 0, 0, target);
        return count;
    }
    public void calculate(int[] nums, int i, int currsum, int target) {
        if (i == nums.length) {
            if (currsum == target)
                count++;
        } else {
            calculate(nums, i + 1, currsum + nums[i], target);
            calculate(nums, i + 1, currsum - nums[i], target);
        }
    }
}