LeetCode671. 二叉树中第二小的节点

题目

纯暴力

class Solution {
public:
    vector<int>ans;
    int findSecondMinimumValue(TreeNode* root) {
        dfs(root);
        sort(ans.begin(),ans.end());
        int res = ans[0];
        for(int i = 1;i < ans.size();i++){
            if(ans[i] != res) return ans[i];
        }
        return -1;
    }
    void dfs(TreeNode* root){
        if(root!=NULL){
            dfs(root->left);
            ans.push_back(root->val);
            dfs(root->right);
        }
    }

};

递归

class Solution {
public:
    
    int findSecondMinimumValue(TreeNode* root) {
        if(!root || !root->left || !root->right)  return -1; //不满足题意
        int left = root->left->val,right = root->right->val;
        if(root->val == root->left->val)  left = findSecondMinimumValue(root->left);
        if(root->val == root->right->val)  right = findSecondMinimumValue(root->right);

        if(root->val == left && root->val == right ) return -1;
        if(root->val < min(left,right)) return min(left,right);
        else return max(left,right);
    }
   
};