C++ 解法, 执行用时: 32ms, 内存消耗: 8112KB, 提交时间: 2022-08-06

/**
 * struct TreeNode {
 *	int val;
 *	struct TreeNode *left;
 *	struct TreeNode *right;
 *	TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 * };
 */
static const auto io_sync_off = [](){
    std::ios::sync_with_stdio(false);
    std::cout.tie(nullptr);
    std::cin.tie(nullptr);
    return nullptr;
}();
class Solution {
public:
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param root TreeNode类 
     * @return int整型
     */
    int minDifference(TreeNode* root) {
        // write code here
        stack<TreeNode*>st;
        TreeNode* c=root;
        TreeNode* p=NULL;
        int r=INT_MAX;
        while(c||!st.empty())
        {
            if(c)
            {
                st.push(c);
                c=c->left;
            }
            else{
                c=st.top();
                st.pop();
                if(p)
                    r=min(r,c->val-p->val);
                p=c;
                c=c->right;
            }
        }
        return r;
    }
};

C++ 解法, 执行用时: 33ms, 内存消耗: 8080KB, 提交时间: 2022-05-29

static const auto io_sync_off = [](){
    std::ios::sync_with_stdio(false);
    std::cout.tie(nullptr);
    std::cin.tie(nullptr);
    return nullptr;
}();
class Solution {
public:
    int minDifference(TreeNode* root) {
        stack<TreeNode*>st;
        TreeNode* c=root;
        TreeNode* p=NULL;
        int r=INT_MAX;
        while(c||!st.empty())
        {
            if(c)
            {
                st.push(c);
                c=c->left;
            }
            else
            {
                c=st.top();
                st.pop();
                if(p)
                    r=min(r,c->val-p->val);
                p=c;
                c=c->right;
            }
        }
        return r;
    }
};

C++ 解法, 执行用时: 45ms, 内存消耗: 8784KB, 提交时间: 2022-06-04

/**
 * struct TreeNode {
 *	int val;
 *	struct TreeNode *left;
 *	struct TreeNode *right;
 *	TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 * };
 */
class Solution {
public:
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param root TreeNode类 
     * @return int整型
     */
    int res = 1000000000;
    int lastVal = 0;
    int count = 0;
    
    int minDifference(TreeNode* root) {
        travel(root);
        return res;
    }
    
    void travel(TreeNode* root) {
        if (root == NULL) {
            return;
        }
        travel(root->left);
        if (count > 0) {
            res = root->val - lastVal > res ? res : root->val - lastVal;
        }
        count++;
        lastVal = root->val;
        travel(root->right);
    }
};

C++ 解法, 执行用时: 45ms, 内存消耗: 8804KB, 提交时间: 2022-06-23

/**
 * struct TreeNode {
 *	int val;
 *	struct TreeNode *left;
 *	struct TreeNode *right;
 *	TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 * };
 */
class Solution {
public:
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param root TreeNode类 
     * @return int整型
     */
    int ret = INT_MAX;
    TreeNode* pre =NULL;
    int minDifference(TreeNode* root) {
        // write code here
//         findMinDiff(root, NULL);
        findMinDiff(root);
        return ret;
    }
    
    void findMinDiff(TreeNode* root){
        if(!root) return;
//         if(from) ret = min(ret, abs(from->val - root->val));
//         pre = root;
        findMinDiff(root->left);
        if(pre) {
            ret = min(ret, abs(pre->val - root->val));
        }
        pre = root;
        findMinDiff(root->right);
    }
};

C 解法, 执行用时: 45ms, 内存消耗: 8820KB, 提交时间: 2022-07-13

void dfs(struct TreeNode* root, int* f, int* a) {
    if (root == NULL) {
        return;
    }

    dfs(root->left, f, a);
    if (*f == -1) {
        *f = root->val;
    } else {
        *a = fmin(*a, root->val - *f);
        *f = root->val;
    }
    dfs(root->right, f, a);
}

int minDifference(struct TreeNode* root ) {
    int a = 1000000000;
    int f = -1;
    dfs(root, &f, &a);
    return a;
}