从上到下按层打印二叉树，同一层的节点按从左到右的顺序打印，每一层打印到一行。 例如:
给定二叉树: [3,9,20,null,null,15,7] 返回其层次遍历结果：
[ [3], [9,20], [15,7] ]
/** * Definition for a binary tree node. * struct TreeNode { * * int val; * * struct TreeNode *left; * * struct TreeNode *right; * * }; * */ * /** * Return an array of arrays of size * *returnSize. * * The sizes of the arrays are returned as * *returnColumnSizes array. * * Note: Both returned array and * *columnSizes array must be malloced, assume caller calls free(). * */ #define MAXSIZE 10000 int** levelOrder(struct TreeNode* root, int* returnSize, int** returnColumnSizes) { if(NULL == root) { *returnSize = 0; return NULL; } struct TreeNode *queue[MAXSIZE] = {0}; //申请队列 int head = 0, tail = 0; //定义头尾指针 int **res = (int**)malloc(sizeof(int*)*MAXSIZE);//申请二维数组 *returnColumnSizes = (int*)malloc(sizeof(int)*MAXSIZE);//申请一维数组 queue[tail++] = root;//将根节点入队 *returnSize = 0; while(head < tail) { int size = (tail - head)%MAXSIZE;获取对内元素个数 (*returnColumnSizes)[*returnSize] = size; //一维数组中存放元素个数 res[*returnSize] = (int*)malloc(sizeof(int)*size);//依据元素个数开辟行数组 for(int i = 0; i < size; ++i) { struct TreeNode *temp = NULL; temp = queue[head++]; res[*returnSize][i] = temp->val; if(temp->left) { queue[tail++] = temp->left; } if(temp->right) { queue[tail++] = temp->right; } (*returnSize)++; } return res; }