Leetcode Binary Tree Postorder Traversal(面试题推荐)

此题来自leetcode   https://oj.leetcode.com/problems/binary-tree-postorder-traversal/

Given a binary tree, return the postorder traversal of its nodes' values.

For example:

Given binary tree {1,#,2,3},

   1
    \
     2
    /
   3

return [3,2,1].

Note: Recursive solution is trivial, could you do it iteratively?

   非递后续归遍历二叉树，肯定得用到栈。先序遍历很好写，但后续遍历就不是那么容易了。

   只需要设置个指针pre，指向最后输出的那个节点就行了，只要判断cur指针指向的是上次输出节点的父节点，且cur无其他未遍历的节点，这个时候就把cur节点输出即可，然后更改pre。原理是要遍历当前节点，其所有子节点都必须遍历完，因为肯定是先左后右，所以只需一个指针保持前一次输出的结果即可。

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> postorderTraversal(TreeNode *root) {
        vector<int> v;
        if (NULL == root)
            return v;
        stack<TreeNode*> s;
        s.push(root);
        TreeNode * pre = root;
        while (!s.empty()) {
            TreeNode * cur = s.top();
            if ((NULL == cur->left && NULL == cur->right) || (pre == cur->left || pre == cur->right)) {
                s.pop();
                v.push_back(cur->val);
                pre = cur;
            }
            else {
                if (cur->right)
                    s.push(cur->right);
                if (cur->left)
                    s.push(cur->left);
            }
        }
        return v;
    }
};