leetcode 494. Target Sum

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于 2019-09-22 21:19:11 发布

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本文介绍了一个算法问题TargetSum，目标是在给定的整数列表中，通过选择加号或减号来达到目标和S的方法数量。示例展示了当输入为[1,1,1,1,1]且目标为3时，存在5种方式实现目标和。文章包含了一个C++类解决方案，使用递归方法解决此问题。

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Target Sum

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.
Example 1:
Input: nums is [1, 1, 1, 1, 1], S is 3.
Output: 5
Explanation:

-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

There are 5 ways to assign symbols to make the sum of nums be target 3.

class Solution {
public:
    int findTargetSumWays(vector<int>& nums, int S) {
        
        return helper(nums, S, 0, 0);
    }
    int helper(vector<int>& nums, int S, int cur, int start){
        if(start == nums.size()){
            if(cur == S){
                return 1;
            } else{
                return 0;
            }
            
        }
        return helper(nums, S, cur + nums[start], start + 1) + helper(nums, S, cur - nums[start], start + 1);
        
    }

};