非递归前序遍历

 京东二面笔试题

/*
    typedef struct {
        int val;
        node* left;
        node* right; 
    } node;
        1
    2      3
4     5  6  7
*/
　　// 刚开始用的vector前插实现, 面试官说用栈实现一下.
    void pre_order(node*  root) {
        if (root == NULL) {
            return;
        }
        vector<node*> vec;
        vec.push_back(root);

        while (!vec.empty()) {
            node* cur = vec.begin();
            vec.erase(vec.begin());
            cout << cur->val << endl; 
            if (cur.right) {
                vec.push_back(vec.begin(), cur.right);
            }
            if (cur.left) {
                vec.push_back(vec.begin(), cur.left);
            }
        }
    }
/*
        1
    2      3
4      5  6  7
*/
　　// 写着写着发现一样的逻辑
    void pre_order(node* root) {
        if (root == NULL) {
            return;
        }
        stack<node*> stk;
        stk.push(root);
        while (!stk.empty()) {
            node *cur = stk.top();
            stk.pop();
            cout << cur->val << endl;
            if (cur->right) {
                stk.push(cur->right);
            }

            if (cur->left) {
                stk.push(cur->left);
            }
        }

        return;
    }