Add Solution for 102 Binary Tree Level Order Traversal

cpp/102_Binary_Tree_Level_Order_Traversal.cpp

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+// 102. Binary Tree Level Order Traversal
+/**
+ * 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]
+ * ]
+ * 
+ * Tags: Tree, Breadth-first Search
+ * 
+ * Similar Problems: (M) Binary Tree Zigzag Level Order Traversal, (E) Binary Tree Level Order Traversal II, 
+ * (E) Minimum Depth of Binary Tree, (M) Binary Tree Vertical Order Traversal
+ * 
+ * Author: Kuang Qin
+ */ 
+
+#include <iostream>
+#include <vector>
+#include <queue>
+
+using namespace std;
+
+/**
+ * Definition for a binary tree node.
+ */
+struct TreeNode {
+    int val;
+    TreeNode *left;
+    TreeNode *right;
+    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
+    TreeNode(int x, TreeNode* l, TreeNode* r) : val(x), left(l), right(r) {}
+};
+
+// bfs solution
+// time: O(n), space: O(n) - queue
+class Solution {
+public:
+    vector<vector<int>> levelOrder(TreeNode* root) {
+        vector<vector<int>> nodes;
+        if (root == NULL) {
+            return nodes;
+        }
+
+        queue<TreeNode*> q;
+        q.push(root);
+        while (!q.empty()) {
+            int levelCount = q.size();
+            vector<int> level;
+
+            while (levelCount--) {  // output current level
+                TreeNode *curr = q.front();
+                level.push_back(curr->val);
+                if (curr->left != NULL) {
+                    q.push(curr->left);
+                }
+
+                if (curr->right != NULL) {
+                    q.push(curr->right);
+                }
+
+                q.pop();
+            }
+
+            nodes.push_back(level);
+        }
+
+        return nodes;
+    }
+};
+
+// dfs solution
+// time: O(n), space: O(n) - call stack
+class Solution_DFS {
+    void dfs(TreeNode* root, int depth, vector<vector<int>>& nodes) {
+        if (root == NULL) {
+            return;
+        }
+
+        // create a new vector for a new level
+        if (depth >= nodes.size()) {
+            vector<int> level;
+            nodes.push_back(level);
+        }
+
+        nodes[depth].push_back(root->val);
+        dfs(root->left, depth + 1, nodes);
+        dfs(root->right, depth + 1, nodes);
+        return;
+    }
+public:
+    vector<vector<int>> levelOrder(TreeNode* root) {
+        vector<vector<int>> nodes;
+        dfs(root, 0, nodes);            
+        return nodes;
+    }
+};
+
+int main() {
+    TreeNode node5(7), node4(15), node3(20, &node4, &node5), node2(9), node1(3, &node2, &node3);
+    Solution sol;
+    vector<vector<int>> res = sol.levelOrder(&node1);
+    for (int i = 0; i < res.size(); i++) {
+        for (int j = 0; j < res[i].size(); j++) {
+            cout << res[i][j] << " ";
+        }
+        cout << endl;
+    }
+
+    cin.get();
+    return 0;
+}