[Leetcode][Tree][Binary Tree Level Order Traversal ]

树的层次遍历，比较简单，3个题的做法完全一样，只是在特定的地方对结果进行reverse。

 

1、Binary Tree Level Order Traversal

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<vector<int> > levelOrder(TreeNode *root) {
        vector<vector<int> > result;
        vector<TreeNode *> levelNodes;
        if (root == NULL) {
            return result;
        }
        levelNodes.push_back(root);
        while (levelNodes.size() > 0) {
            vector<TreeNode *> nextLevelNodes;
            vector<int> levelVals;
            for (int i = 0; i < levelNodes.size(); i++) {
                levelVals.push_back(levelNodes[i]->val);
                if (levelNodes[i]->left != NULL) {
                    nextLevelNodes.push_back(levelNodes[i]->left);
                }
                if (levelNodes[i]->right != NULL) {
                    nextLevelNodes.push_back(levelNodes[i]->right);
                }
            }
            result.push_back(levelVals);
            levelNodes = nextLevelNodes;
        }
        return result;
    }
};

　2、 Binary Tree Level Order Traversal II

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<vector<int> > levelOrderBottom(TreeNode *root) {
        vector<vector<int> > result;
        vector<TreeNode *> levelNodes;
        if (root != NULL) {
            levelNodes.push_back(root);
        }
        while (levelNodes.size() > 0) {
            vector<TreeNode *> next;
            vector<int> nodeValues;
            for (int i = 0; i < levelNodes.size(); i++) {
                nodeValues.push_back(levelNodes[i]->val);
                if (levelNodes[i]->left != NULL) {
                    next.push_back(levelNodes[i]->left);
                }
                if (levelNodes[i]->right != NULL) {
                    next.push_back(levelNodes[i]->right);
                }
            }
            result.push_back(nodeValues);
            levelNodes = next;
        }
        reverse(result.begin(), result.end());
        return result;
    }
};

　3、Binary Tree Zigzag Level Order Traversal

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<vector<int> > zigzagLevelOrder(TreeNode *root) {
        vector<vector<int> > result;
        vector<TreeNode *> nodes;
        if (root != NULL) {
            nodes.push_back(root);
        }
        bool flag = false;
        while (nodes.size() > 0) {
            vector<TreeNode *> next;
            vector<int> values;
            for (int i = 0; i < nodes.size(); i++) {
                values.push_back(nodes[i]->val);
                if (nodes[i]->left != NULL) {
                    next.push_back(nodes[i]->left);
                }
                if (nodes[i]->right != NULL) {
                    next.push_back(nodes[i]->right);
                }
            }
            if (flag) {
                reverse(values.begin(), values.end());
            }
            result.push_back(values);
            nodes = next;
            flag = !flag;
        }
        return result;
    }
};