Image Overlap LeetCode solution in C++

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class Solution {
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 public:
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  int largestOverlap(vector<vector<int>> &img1, vector<vector<int>> &img2) {
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    int n = img1.size();
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    vector<pair<int, int>> vec_a;
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    vector<pair<int, int>> vec_b;
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    for (int i = 0; i < n; i++) {
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      for (int j = 0; j < n; j++) {
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        if (img1[i][j] == 1) {
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          vec_a.push_back({i, j});
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        }
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        if (img2[i][j] == 1) {
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          vec_b.push_back({i, j});
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        }
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      }
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    }
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    int ans = 0;
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    map<pair<int, int>, int> mp;
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    for (auto [i1, j1] : vec_a) {
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      for (auto [i2, j2] : vec_b) {
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        mp[{i1 - i2, j1 - j2}]++;
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        ans = max(ans, mp[{i1 - i2, j1 - j2}]);
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      }
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    }
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    return ans;
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  }
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};