Validate Binary Search Tree Solutions in C++

// Source : https://oj.leetcode.com/problems/validate-binary-search-tree/
// Author : Hao Chen
// Date   : 2014-07-05



#include <iostream>
#include <vector>
using namespace std;


struct TreeNode {
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};

bool isValidBST(TreeNode *root) {

    //travel the tree by inner-order
    vector<TreeNode*> stack;
    TreeNode* node = root;
    vector<int> v;
    while (stack.size()>0 || node!=NULL) {
        if (node!=NULL){
            stack.push_back(node);
            node = node->left;
        }else{
            node = stack.back();
            stack.pop_back();
            v.push_back(node->val);
            node = node->right;
        }
    }

    //check the vector wehther sorted or not
    for(int i=0; v.size()>0 && i<v.size()-1; i++){
        if (v[i] >= v[i+1]) {
            return false;
        }
    }

    return true;
}


TreeNode* createTree(int a[], int n)
{
    if (n<=0) return NULL;

    TreeNode **tree = new TreeNode*[n];

    for(int i=0; i<n; i++) {
        if (a[i]==0 ){
            tree[i] = NULL;
            continue;
        }
        tree[i] = new TreeNode(a[i]);
    }
    int pos=1;
    for(int i=0; i<n && pos<n; i++) {
        if (tree[i]){
            tree[i]->left = tree[pos++];
            if (pos<n){
                tree[i]->right = tree[pos++];
            }
        }
    }
    return tree[0];
}


int main()
{
    cout << isValidBST(NULL) << endl;

    int a[]={1,1};
    cout << isValidBST(createTree(a, sizeof(a)/sizeof(int))) << endl;
    
    int b[]={4,2,6,1,7,5,7};
    cout << isValidBST(createTree(b, sizeof(b)/sizeof(int))) << endl;

    int c[]={4,2,6,1,3,5,7};
    cout << isValidBST(createTree(c, sizeof(c)/sizeof(int))) << endl;
    return 0;
}

#include <cstddef>

struct TreeNode {
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};

class Solution {
public:
    bool isValidBST(TreeNode *root) {
        return isValidBST(root, false, false, 0, 0);
    }
    bool isValidBST(TreeNode *root, bool left, bool right, int min, int max)
    {
        if (!root) return true;
        else if ((left && root->val <= min) || (right && root->val >= max)) return false;
        else return isValidBST(root->left, left, true, min, root->val) && isValidBST(root->right, true, right, root->val, max);
    }
};

/// Source : https://leetcode.com/problems/validate-binary-search-tree/description/
/// Author : liuyubobobo
/// Time   : 2018-05-09

#include <iostream>
#include <vector>

using namespace std;


/// Definition for a binary tree node.
struct TreeNode {
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};


/// Using inOrder traverse
/// Store all elements in an vector
///
/// Time Complexity: O(n)
/// Space Complexity: O(n)
class Solution {
public:
    bool isValidBST(TreeNode* root) {

        vector<int> vec;
        inOrder(root, vec);
        for(int i = 1 ; i < vec.size() ; i ++)
            if(vec[i-1] >= vec[i])
                return false;
        return true;
    }

private:
    void inOrder(TreeNode* node, vector<int>& vec){

        if(node == NULL)
            return;

        inOrder(node->left, vec);
        vec.push_back(node->val);
        inOrder(node->right, vec);
    }
};


int main() {

    return 0;
}

/// Source : https://leetcode.com/problems/validate-binary-search-tree/description/
/// Author : liuyubobobo
/// Time   : 2018-05-09

#include <iostream>
#include <vector>

using namespace std;


/// Definition for a binary tree node.
struct TreeNode {
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};

/// Recursive test
/// Time Complexity: O(n)
/// Space Complexity: O(h)
class Solution {
public:
    bool isValidBST(TreeNode* root) {

        return isValidBST(root, INT_MIN, INT_MAX);
    }

private:
    bool isValidBST(TreeNode* node, int min, int max){

        if(node == NULL)
            return true;

        if(node->val < min || node->val > max)
            return false;

        if(node->left != NULL && node->left->val >= node->val)
            return false;

        if(node->right != NULL && node->right->val <= node->val)
            return false;

        return isValidBST(node->left, min, node->val - 1) && isValidBST(node->right, node->val + 1, max);
    }
};


int main() {

    return 0;
}

/// Source : https://leetcode.com/problems/validate-binary-search-tree/description/
/// Author : liuyubobobo
/// Time   : 2018-12-16

#include <iostream>
#include <stack>

using namespace std;


/// Definition for a binary tree node.
struct TreeNode {
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};


/// Non-Recursion Solution for Recursion thinking
/// Time Complexity: O(n)
/// Space Complexity: O(h)
class Solution {

public:
    bool isValidBST(TreeNode* root) {

        if(!root) return true;

        stack<TreeNode*> st;
        stack<int> upper, lower;

        st.push(root);
        upper.push(INT_MAX);
        lower.push(INT_MIN);
        while(!st.empty()){
            TreeNode* cur = st.top();
            st.pop();

            int left = lower.top();
            lower.pop();

            int right = upper.top();
            upper.pop();

            if(cur->val > right || cur->val < left)
                return false;

            if(cur->right){
                if(cur->right->val <= cur->val) return false;
                st.push(cur->right);
                lower.push(cur->val + 1);
                upper.push(right);
            }

            if(cur->left){
                if(cur->left->val >= cur->val) return false;
                st.push(cur->left);
                lower.push(left);
                upper.push(cur->val - 1);
            }
        }
        return true;
    }
};


int main() {

    return 0;
}

/// Source : https://leetcode.com/problems/validate-binary-search-tree/description/
/// Author : liuyubobobo
/// Time   : 2018-05-22

#include <iostream>

using namespace std;


/// Definition for a binary tree node.
struct TreeNode {
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};


/// InOrder traverse
/// validate during the traversing
///
/// Time Complexity: O(n)
/// Space Complexity: O(h)
class Solution {

private:
    int pre = -1;
    bool isPre = false;

public:
    bool isValidBST(TreeNode* root) {

        isPre = false;
        pre = -1;
        return inOrder(root);
    }

private:
    bool inOrder(TreeNode* node){

        if(node == NULL)
            return true;

        if(!inOrder(node->left))
            return false;

        if(isPre && pre >= node->val)
            return false;
        isPre = true;
        pre = node->val;

        if(!inOrder(node->right))
            return false;

        return true;
    }
};


int main() {

    return 0;
}

/// Source : https://leetcode.com/problems/validate-binary-search-tree/description/
/// Author : liuyubobobo
/// Time   : 2018-05-28

#include <iostream>

using namespace std;


/// Definition for a binary tree node.
struct TreeNode {
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};


/// Morris InOrder traversal
/// Attention: you can not change the give Tree structure in Leetcode,
/// So try to return result early will lead to RE :-)
///
/// Time Complexity: O(n)
/// Space Complexity: O(1)
class Solution {

private:
    bool flag;
    int pre;

public:
    bool isValidBST(TreeNode* root) {

        flag = false;
        pre = -1;
        TreeNode* cur = root;

        bool res = true;
        while(cur != NULL){
            if(cur->left == NULL){
                if(!process(cur))
                    res = false;
                cur = cur->right;
            }
            else{
                TreeNode* prev = cur->left;
                while(prev->right != NULL && prev->right != cur)
                    prev = prev->right;

                if(prev->right == NULL){
                    prev->right = cur;
                    cur = cur->left;
                }
                else{
                    prev->right = NULL;
                    if(!process(cur))
                        res = false;
                    cur = cur->right;
                }
            }
        }

        return res;
    }

private:
    bool process(TreeNode* node){

        if(flag && pre >= node->val)
            return false;
        flag = true;
        pre = node->val;
        return true;
    }
};


void print_bool(bool res){
    cout << (res ? "True" : "False") << endl;
}

int main() {

    TreeNode* root = new TreeNode(5);
    root->left = new TreeNode(1);
    root->right = new TreeNode(4);
    root->right->left = new TreeNode(3);
    root->right->right = new TreeNode(6);

    print_bool(Solution().isValidBST(root));

    return 0;
}