面试题11：二叉树的非递归前、中、后、层级遍历

#include <vector>
#include <stack>
using std::vector;

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

vector<int> preOrder(TreeNode* root) {
    vector<int> res;
    if (root == nullptr)
        return res;

    std::stack<TreeNode*> s;
    TreeNode* p = root;
    while (p || !s.empty()) {
        while (p) {
            res.push_back(p->val);
            s.push(p);
            p = p->left;
        }

        if (!s.empty()) {
            p = s.top();
            s.pop();
            p = p->right;
        }
    }
    return res;
}

vector<int> inOrder(TreeNode* root) {
    vector<int> res;
    if (root == nullptr)
        return res;
    TreeNode* p = root;
    std::stack<TreeNode*> s;

    while (p || !s.empty()) {

        while (p) {
            s.push(p);
            p = p->left;
        }

        if (!s.empty()) {
            p = s.top();
            s.pop();
            res.push_back(p->val);
            p = p->right;
        }
    }
    return res;
}

vector<int> postorderTraversal(TreeNode* root) {
    vector<int> res;
    if (root == nullptr)
        return res;

    std::stack<TreeNode*> s;
    TreeNode* cur = root;
    TreeNode* pre = nullptr;

    while (cur) {
        s.push(cur);
        cur = cur->left;
    }

    while (!s.empty()) {
        cur = s.top();
        s.pop();
        if (cur->right == nullptr || cur->right == pre) {
            res.push_back(cur->val);
            pre = cur;
        } else {
            s.push(cur);
            cur = cur->right;
            while (cur) {
                s.push(cur);
                cur = cur->left;
            }
        }
    }
    return res;
}