LC968 WA

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
    using pii = pair<int, int>;
    using a3 = array<int, 3>;
    pii dfs(TreeNode* node) {
        // cost of subtree covered exculte itself,
        // cost of subtree covered all
        pii ret = {0, 1};
        auto child = vector<TreeNode*>{node->left, node->right};
        for (auto v: child) {
            auto [x, y] = dfs(v);

        }
        return ret;
    }
    pii dfs1(TreeNode* node) {
        // cover by itself, cover by its child
        pii ret = {1, INT_MAX / 2};
        int min_delta = INT_MAX / 2;
        int child_cost = 0;
        auto child = vector<TreeNode*>{node->left, node->right};
        for (auto v: child) {
            if (v == nullptr) continue;
            auto [self_cover, child_cover] = dfs1(v);
            child_cost += min(self_cover, child_cover);
            min_delta = min(min_delta, child_cover - self_cover);
        }
        ret.first += child_cost;
        ret.second += child_cost + (min_delta < 0 ? 0 : min_delta);
        return ret;
    }
    a3 dfs2(TreeNode* node) {
        a3 ret = {0, INT_MAX / 2, 1};
        auto child = vector<TreeNode*>{node->left, node->right};
        int min_delta = INT_MAX / 2;
        for (auto v: child) {
            if (v == nullptr) continue;
            auto [subtree, cover_noplace, cover_place] = dfs2(v);
            ret[2] += min({subtree, cover_noplace, cover_place});
            ret[1] += min(cover_noplace, cover_place);
            min_delta = min(min_delta, cover_place - cover_noplace);
            ret[0] += cover_noplace;
        }
        if (min_delta > 0) ret[1] += min_delta;
        return ret;
    }
public:
    int minCameraCover(TreeNode* root) {
        auto ans = dfs2(root);
        return min(ans[1], ans[2]);
    }
};