Dfs

DFS/ValidateBinarySearchTree.cpp

@@ -0,0 +1,37 @@
+/**
+ * Definition for a binary tree node.
+ * struct TreeNode {
+ *     int val;
+ *     TreeNode *left;
+ *     TreeNode *right;
+ *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
+ * };
+ */
+class Solution {
+public:
+    bool isValidBST(TreeNode* root)
+    {
+        stack<TreeNode*> s;
+        TreeNode* pre = NULL;
+
+        while(root || !s.empty())
+        {
+            while(root)
+            {
+                s.push(root);
+                root = root-> left;
+
+            }
+            root = s.top(), s.pop();
+
+            if(pre != NULL && root-> val <= pre-> val)
+                return false;
+            pre = root;
+            root=root->right;
+
+        }
+        return true;
+    }
+};

DP/00-Guide.md

@@ -15,4 +15,9 @@
 *List of Questions Added*
 
 1. fibonacci seq
-1. ways to nth step
+2. ways to nth step
+3. edit distance
+4. Increasing Sequence
+5. Best Time to Buy and Sell Stock II
+
+

DP/IncreasingSequence.cpp → DP/4-IncreasingSequence.cpp

@@ -1,4 +1,6 @@
 #include <bits/stdc++.h>
+#include<math.h>
+#include<iostream>
 using namespace std;
 
 /* lis() returns the length of the longest

DP/5-BestTimeToBuyandSellStockII.cpp

@@ -0,0 +1,10 @@
+class Solution {
+public:
+    int maxProfit(vector<int>& P) {
+ int profit = 0;
+        for(int i = 1; i < size(P); i++)
+            if(P[i] > P[i-1])              // yesterday was valley, today is peak
+                profit += P[i] - P[i-1];   // buy yesterday, sell today
+        return profit;
+    }
+};