heap-buffer-overflow on 3Sum implementation

Question

I'm new to c++ and I'm trying to resolve the problem of 3Sum in LeetCode. However, I can't deal with this error which is called "heap-buffer-overflow".

vector<vector<int>> threeSum(vector<int>& nums) {
    vector<vector<int>> res;
    int numslen = nums.size();
    sort(nums.begin(), nums.end());
    int i = 0;
    while(i < numslen - 2){
        int start = i + 1;
        int end = numslen - 1;
        while (start < end) {
            if (nums[i] + nums[start] + nums[end] == 0) {
                res.push_back({ nums[i], nums[start], nums[end] });
                end--;
                while (nums[end] == nums[end+1]) { end--;   }
            }
            else if (nums[i] + nums[start] + nums[end] > 0) { end--; }
            else { start++; }
        }
        i++;
        while (nums[i] == nums[i - 1]) { i++;   }
    }   
    return res;
}

This is the error message.

AddressSanitizer: heap-buffer-overflow on address 0x60200000044c at pc 0x000000406c74 bp 0x7ffd2a1cd0d0 sp 0x7ffd2a1cd0c8

Watch out for overstepping the bounds with i and end. Suppose, for instance, that your input is a vector with three identical numbers... — molbdnilo
– molbdnilo, Commented May 17, 2019 at 12:06
And please show us how you called this function when you got that error! — AKL
– AKL, Commented May 17, 2019 at 12:08

UmNyobe · Accepted Answer · 2019-05-17 12:20:55Z

0

while (nums[i] == nums[i - 1]) { i++;}

should be

while (i < numslen && nums[i] == nums[i - 1]) { i++;}

The first time the loop above is executed, i and i-1 are guaranteed to be within range. But what if the nums have trailing duplicate numbers ? Your version will read "forever" on invalid memory.

edited May 17, 2019 at 12:20

answered May 17, 2019 at 12:09

UmNyobe

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4 Comments

Kong John Over a year ago

Thanks for your anwser. But there is another bug I just found that can also make this wrong message. while (nums[end] == nums[end+1]) { end--;} should be while (start < end && nums[end] == nums[end+1]) { end--;}.

AKL Over a year ago

@KongJohn I am getting no error when I run your code for a small vector. Could you please show us how you call the function (specially the input vector)?

UmNyobe Over a year ago

@KongJohn yes both are similar.

Kong John Over a year ago

@AKL You can try following code:vector<vector<int>> res; vector<int> nums = {0,0,0}; res = threeSum(nums);, and you will realize the bug.

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