[LeetCode] Binary Tree Level Order Traversal

★ 题目

https://leetcode.com/problems/binary-tree-level-order-traversal/description/

Given a binary tree, return the level order traversal of its nodes’ values. (ie, from left to right, level by level).

For example:
Given binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its level order traversal as:

[
  [3],
  [9,20],
  [15,7]
]

★ 代码

34 / 34 test cases passed.
Runtime: 5 ms

/**
 * 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 *columnSizes array.
 * Note: Both returned array and *columnSizes array must be malloced, assume caller calls free().
 */

#define max(a,b) (((a) > (b))?(a):(b));

int getLevels(struct TreeNode* node) {
    if (node == NULL) return 0;
    int level1 = 0;
    if (node->left) {
        level1 = getLevels(node->left);
    }
    int level2 = 0;
    if (node->right) {
        level2 = getLevels(node->right);
    }
    return 1 + max(level1, level2);
}

// level：当前正在处理哪一层
void getEachLevelSize(struct TreeNode* node, int level, int levelCnt, int levelSizes[]) {
    if (level >= levelCnt) return;
    if (node == NULL) return;
    levelSizes[level]++;
    if (node->left != NULL) {
        getEachLevelSize(node->left, level+1, levelCnt, levelSizes);
    }
    if (node->right != NULL) {
        getEachLevelSize(node->right, level+1, levelCnt, levelSizes);
    }
}

// level：当前正在处理哪一层
void traverse(struct TreeNode* node, int level, int levelCnt, int *levelSizes, int** result) {
    if (level >= levelCnt) return;
    if (node == NULL) return;
    result[level][levelSizes[level]++] = node->val;
    if (node->left != NULL) {
        traverse(node->left, level+1, levelCnt, levelSizes, result);
    }
    if (node->right != NULL) {
        traverse(node->right, level+1, levelCnt, levelSizes, result);
    }
}

int** levelOrder(struct TreeNode* root, int** columnSizes, int* returnSize) {    
    int levelCnt = getLevels(root);
    *returnSize = levelCnt;

    // levelSizes： 每一层数据的个数
    int *levelSizes = (int*)malloc(levelCnt * sizeof(int));
    for (int i = 0; i < levelCnt; i++) {
        levelSizes[i] = 0;
    }
    getEachLevelSize(root, 0, levelCnt, levelSizes);

    int** result = (int**)malloc(levelCnt*sizeof(int*));
    for(int i = 0; i < levelCnt; i++) {
        //printf("levelSizes[%d]: %d\n", i, levelSizes[i]);
        int size = levelSizes[i] * sizeof(int);
        result[i] = (int*)malloc(size);
        memset(result[i], 0, size);
    }
    for (int i = 0; i < levelCnt; i++) {
        levelSizes[i] = 0;
    }
    traverse(root, 0, levelCnt, levelSizes, result);
    *columnSizes = levelSizes;
    return result;
}