adds left view of binary search tree

Author: bvishal8510

Trees/Binary_Tree_Left_View.cpp

@@ -0,0 +1,161 @@
+/*
+    The Left View of a Binary Tree depicts the nodes that are visible 
+    when the tree is viewed from the left side of it. At every level 
+    there would be exactly one node that will appear in left view which
+    would be the first node of that level.
+*/
+
+#include <bits/stdc++.h>
+using namespace std;
+#define mkp make_pair
+
+// Node of Binary Tree storing data, level,
+// left and right child information
+struct Node 
+{
+    int data;
+    int level;
+    Node *left, *right;
+    Node(int val) 
+    {
+        data = val;
+        left = NULL;
+        right = NULL;
+    }
+};
+
+// Function to print Left View of Binary Tree
+void leftView(Node *root) 
+{
+    if (root == NULL)
+        return;
+
+    // initialising variables
+    queue<Node *> q;
+    q.push(root);
+    root -> level = 0;
+    map<int, int> mp;
+
+    // variable to store level of nodes
+    int level;
+
+    // asigning level to each node of Binary Tree
+    // and replacing nodes of same level in a map with
+    // key as level so as to obtain the Left View
+    while (!q.empty()) 
+    {
+        // extract the node at the front of queue
+        Node *temp = q.front();
+        level = temp -> level;
+
+        // make key as level and data as value for map
+        if(mp.find(level) == mp.end())
+            mp[level] = temp -> data;
+
+        // remove the extract node from queue
+        q.pop();
+
+        // when left child exists, assign level to it,
+        // and push it to the queue
+        if (temp -> left != NULL) 
+        {
+            temp -> left -> level = level + 1;
+            q.push(temp -> left);
+        }
+
+        // when right child exists, assign level to it,
+        // and push it to the queue
+        if (temp -> right != NULL) 
+        {
+            temp -> right -> level = level + 1;
+            q.push(temp -> right);
+        }
+    }
+    /* 
+        Map mp contains:
+        [0] : {1}
+        [1] : {2}
+        [2] : {4}
+        [3] : {8}   
+    */
+
+    cout << "Left View of Binary Tree: " << endl;
+    map<int, int> :: iterator it;
+    // Iterate over the map keys i.e 0, 1, 2, 3
+    for (it = mp.begin(); it != mp.end(); it++)
+        cout << it -> second << " ";
+}
+
+// Driver Function
+int main() 
+{
+    map<int, Node *> m;
+    // Input number of edges
+    int n;
+    cin >> n;
+    Node *root = NULL;
+
+    /*
+        Input Format:
+        3
+        1 2 L
+        1 3 R
+        2 4 L
+        This means there are 3 edges
+        2 is the left child of 1,
+        3 is the right child of 1,
+        4 is the left child of 2.
+    */
+    for (int i = 0; i < n; i++) 
+    {
+        int node1, node2;
+        char direction;
+        cin >> node1 >> node2 >> direction;
+        Node *parent, *child;
+        if (m.find(node1) == m.end())
+        {
+            parent = new Node(node1);
+            m[node1] = parent;
+            if (root == NULL)
+                root = parent;
+        }
+        else
+            parent = m[node1];
+        child = new Node(node2);
+        if (direction == 'L')
+            parent -> left = child;
+        else
+            parent -> right = child;
+        m[node2] = child;
+    }
+
+    // call to leftView function
+    leftView(root);
+    return 0;
+}
+
+/*
+    Input:
+    8
+    1 2 L
+    1 3 R
+    2 4 L
+    2 5 R
+    3 6 L
+    3 7 R
+    5 8 L
+    6 9 R
+    
+            1
+         /     \
+        2       3
+      /   \   /   \
+    4      5 6     7
+         /   \
+        /      \
+       8        9
+
+    Output:
+    Left View of Binary Tree:
+    1 2 4 8
+*/