Careercup

2014-05-03 23:35

题目链接

原题：

For a given node in binary search tree find a next largest number in search tree.

题目：给定一个二叉搜索树的节点，找出此节点在树中的中序后继节点，也就是比它大的最小节点。

解法：右子树向左走到底，左祖先走到顶向右。草稿纸上一画图就清楚了。

代码：

// http://www.careercup.com/question?id=5205167846719488
struct TreeNode {
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode(int _val = 0): val(_val), left(nullptr), right(nullptr) {};
};

class Solution {
public:
    int nextLargestNumber(TreeNode *root, int val) {
        if (root == nullptr) {
            // nothing to do
            return val;
        }
        
        left_top = nullptr;
        return findRecursive(root, val);
    }
private:
    TreeNode *left_top;
    
    int findRecursive(TreeNode *root, int val) {
        if (root == nullptr) {
            // not found, return val
            return val;
        } else if (root->val > val) {
            left_top = root;
            return findRecursive(root->left, val);
        } else if (root->val < val) {
            return findRecursive(root->right, val);
        } else {
            if (left_top == nullptr) {
                // val is the greatest of all.
                return val;
            }
            if (root->right == nullptr) {
                return left_top->val;
            }
            
            left_top = root->right;
            while (left_top->left != nullptr) {
                left_top = left_top->left;
            }
            return left_top->val;
        }
    };
}