LeetCode报错：Line 1034：Char9

报错信息：Line 1034: Char 9: runtime error: reference binding to null pointer of type ‘std::vector<int, std::allocator>’ (stl_vector.h)
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /usr/bin/…/lib/gcc/x86_64-linux-gnu/9/…/…/…/…/include/c++/9/bits/stl_vector.h:1043:9

class Solution {public:vector<vector<int>> subsets(vector<int>& nums) {vector<vector<int>> ans;vector<int> temp;ans[0]={};//这样的赋值方式是错误的for(int i=0;i<nums.size();i++){int last_ans_size=ans.size();for(int j=0;j<last_ans_size;j++){temp=ans[j];temp.push_back(nums[i]);ans.push_back(temp);}}return ans;}
};

错误分析：ans初始化后可能为空，直接用下标索引的话会导致空指针错误nullptr。
用vs跑这个代码会发生vector越界错误，验证了上述的错误分析。

int main() {vector<vector<int>> ve;ve[0] = {};//错误cout << ve.size() << endl;cout << ve[0].size();return 0;
}

正确的赋值方式应为

int main() {vector<vector<int>> ve;ve.push_back({});//正确的赋值方式ve[0] = { 1 };//给ve赋值后就可以用下标访问ve[0]了cout << ve.size() << endl;cout << ve[0].size();return 0;
}

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