C++ 栈的中序遍历

C++ 栈利用 时间复杂度最大O(N)
从根节点开始入栈，出栈方向就是中序遍历。

/*
struct TreeNode {
    int val;
    struct TreeNode *left;
    struct TreeNode *right;
    TreeNode(int x) :
            val(x), left(NULL), right(NULL) {
    }
};
*/
class Solution {
public:
    TreeNode* KthNode(TreeNode* pRoot, int k) {
        if(pRoot == nullptr || k == 0) return nullptr;
        stack<TreeNode*> S;
        int count = 0;
        TreeNode* res = pRoot;
        while(res != nullptr || !S.empty()) {
            if(res != nullptr) {
                S.push(res);
                res = res->left;
            } else {
                res = S.top();
                S.pop();
                if(++count == k) return res;
                res = res->right;
            }
        }
        return nullptr;
    }
};