[leetcode]145. 二叉树的后序遍历

1.题目：
给定一个二叉树，返回它的 后序 遍历。

示例:
输入: [1,null,2,3]  
   1
    \
     2
    /
   3 

输出: [3,2,1]

2.代码：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */
/**
 * Return an array of size *returnSize.
 * Note: The returned array must be malloced, assume caller calls free().
 */
typedef struct TreeNode* Tree;
typedef struct{
    Tree* array;
    int top;
}Stack;

int* postorderTraversal(struct TreeNode* root, int* returnSize) {
    *returnSize=0;
    int *r=malloc(1000*sizeof(int));
    Stack* s=(Stack *)malloc(sizeof(Stack));
    s->array=(Tree *)malloc(1000*sizeof(Tree));
    s->top=0;
    Tree t=NULL;									//最近访问节点	
    while(root||s->top){					
        if(root){            
            s->array[s->top++]=root;          	    //push()
            root=root->left;        	    	    //->left
        }
        else{
            root=s->array[s->top-1];
            if(root->right&&root->right!=t){
                root=root->right;
                //s->array[s->top++]=root;          //push()
                //root=root->left;
            }
            else{
                root=s->array[--s->top];       	    //pop()
                r[(*returnSize)++]=root->val;       //visit()                
                t=root;
                root=NULL;
            }            
        }
    }
    return r;
}

3.知识点：

二叉树后序遍历，标记最近访问节点。