Binary Tree Level Order Traversal II

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#BFS #Tree #Javascript #Leetcode #Easy

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本文介绍了一种经典的广度优先搜索（BFS）算法应用——二叉树的层次遍历，并通过一个具体示例展示了如何从叶节点到根节点进行层次遍历，最后返回按层次排列的节点值。

Given a binary tree, return the bottom-up level order traversal of its nodes' values. (ie, from left to right, level by level from leaf to root).

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

return its bottom-up level order traversal as:

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

Tags

Tree, BFS

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*** We need to separate the current level and current level data for returning the result.

Classic BFS question.

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[][]}
 */
var levelOrderBottom = function (root) {
    if (root === null) {
        return [];
    }

    var ret = [];
    var lastLevel = [root];

    while (lastLevel.length > 0) {
        var lastLevelData = [];
        var currentLevel = [];

        for (var i in lastLevel) {
            lastLevelData.push(lastLevel[i].val);

            if (lastLevel[i].left !== null) {
                currentLevel.push(lastLevel[i].left);
            }

            if (lastLevel[i].right !== null) {
                currentLevel.push(lastLevel[i].right);
            }
        }
        ret.unshift(lastLevelData);
        lastLevel = currentLevel;
    }

    return ret;
};