107. 二叉树的层次遍历 II

Posted 2020-12-10 jesseywang

题目描述： 给定一个二叉树，返回其节点值自底向上的层次遍历。 （即按从叶子节点所在层到根节点所在的层，逐层从左向右遍历）
例如：
给定二叉树 [3,9,20,null,null,15,7],返回其自底向上的层次遍历为： [ [15,7], [9,20], [3] ]

• DFS：递归，从上到下遍历二叉树，每一层都是一个结点数组，每次递归都带上结点所在层数就知道该结点该放在哪里了

//JS

var levelOrderBottom = function(root) {
    let res = [];
    let dfs = (node, depth) => {
        if(!node) return;
        res[depth] = res[depth] || [];
        res[depth].push(node.val);
        dfs(node.left, depth + 1);
        dfs(node.right, depth + 1);
    }
    dfs(root, 0);
    return res.reverse();
};

• BFS：层次遍历，可以用单队列也可以用双队列

//C语言
//单队列
//先计算二叉树的高度，得到返回二维数组长度，再正常遍历二叉树，将每层的结点数组从后往前放到结果数组中

#define MAXSIZE 2000

int maxDepth(struct TreeNode* root){
    if(root == NULL) return 0;
    int lDepth = maxDepth(root -> left),
    rDepth = maxDepth(root -> right);
    return 1 + (lDepth > rDepth ? lDepth : rDepth);
}

int** levelOrderBottom(struct TreeNode* root, int* returnSize, int** returnColumnSizes){
    *returnSize = maxDepth(root);
    if(*returnSize == 0) return NULL;

    int **res = (int **)malloc(sizeof(int *) * (*returnSize));
    *returnColumnSizes = (int **)malloc(sizeof(int *) * (*returnSize));
    struct TreeNode *queue[MAXSIZE], *p;
    int rear = 0, front = 0, idx = *returnSize;
    queue[rear++] = root;
    while(front != rear){
        int len = rear - front;
        res[--idx] = (int *)malloc(sizeof(int) * len);
        (*returnColumnSizes)[idx] = len;
        for(int i = 0; i < len; i++){
            p = queue[front++];
            res[idx][i] = p -> val;
            if(p -> left) queue[rear++] = p -> left;
            if(p -> right) queue[rear++] = p -> right;
        }
    }
    return res;
}

//JS
var levelOrderBottom = function(root) {
    if(!root) return [];
    let queueArr = [], result = [], node = null;
    queueArr.push(root);
    result.push([root.val]);

    while(queueArr.length != 0) {
        let len = queueArr.length, tmpArr = [];
        for(let i = 0; i < len; i++){
            node = queueArr.shift();
            if(node.left) {
                tmpArr.push(node.left.val);
                queueArr.push(node.left);
            }
            if(node.right) {
                tmpArr.push(node.right.val);
                queueArr.push(node.right);
            }
        }
        if(tmpArr.length != 0)
            result.push(tmpArr);        
    }
    return result.reverse();
};