​LeetCode刷题实战548：将数组分割成和相等的子数组

Input: [1,2,1,2,1,2,1]
Output: True
Explanation:
i = 1, j = 3, k = 5. 
sum(0, i - 1) = sum(0, 0) = 1
sum(i + 1, j - 1) = sum(2, 2) = 1
sum(j + 1, k - 1) = sum(4, 4) = 1
sum(k + 1, n - 1) = sum(6, 6) = 1

class Solution {
public:
    bool splitArray(vector<int>& nums) {
        if(nums.size()<7){
            return false;
        }
        int len=nums.size();
        vector<int>pre_sum(len,0);
        //统计前缀和
        pre_sum[0]=nums[0];
        for(int i=1;i            pre_sum[i]=pre_sum[i-1]+nums[i];
        }
        //使用 j 进行分割
        for(int j=3;j-3;++j){
            unordered_set<int> st;
            //统计前半段可能出现的分割值
            for(int i=1;i-1;++i){
                if(pre_sum[i-1]==pre_sum[j-1]-pre_sum[i]){
                    st.insert(pre_sum[i-1]);
                }
            }
            //判断后半段是否能够出现满足要求的 k
            for(int k=j+2;k-1;++k){
                if(pre_sum[k-1]-pre_sum[j]==pre_sum[len-1]-pre_sum[k]&&st.count(pre_sum[k-1]-pre_sum[j])){
                    return true;
                }
            }
        }
        //跳出循环，返回false
        return false;
    }
};