C++中函数内部实现另一个函数 匿名函数？？？

leetcode 802. 找到最终的安全状态

C++代码为，

class Solution {
public:
    vector<int> eventualSafeNodes(vector<vector<int>>& graph) {
        int n = graph.size();
        vector<int> color(n);

        function<bool(int)> safe = [&](int x) {
            if (color[x] > 0) {
                return color[x] == 2;
            }
            color[x] = 1;
            for (int y : graph[x]) {
                if (!safe(y)) {
                    return false;
                }
            }
            color[x] = 2;
            return true;
        };

        vector<int> ans;
        for (int i = 0; i < n; ++i) {
            if (safe(i)) {
                ans.push_back(i);
            }
        }
        return ans;
    }
};