diff --git a/.gitignore b/.gitignore
index 6e1079e..0f28594 100644
--- a/.gitignore
+++ b/.gitignore
@@ -101,3 +101,6 @@ ENV/
 .mypy_cache/
 
 .vscode
+
+# files
+*.txt
\ No newline at end of file
diff --git a/algs4/depth_first_paths.py b/algs4/depth_first_paths.py
index 14caf6a..b538b15 100644
--- a/algs4/depth_first_paths.py
+++ b/algs4/depth_first_paths.py
@@ -9,7 +9,7 @@
    Run depth first search on an undirected graph.
    Runs in O(E + V) time.
 
-   %  python grapy.py tinyCG.txt
+   %  python graph.py tinyCG.txt
    6 8
    0: 2 1 5
    1: 0 2
@@ -60,6 +60,7 @@ def path_to(self, v):
         path.push(s)
         return path
 
+
 if __name__ == '__main__':
     import sys
     f = open(sys.argv[1])
diff --git a/algs4/nonrecursive_depth_first_paths.py b/algs4/nonrecursive_depth_first_paths.py
new file mode 100644
index 0000000..ed15f96
--- /dev/null
+++ b/algs4/nonrecursive_depth_first_paths.py
@@ -0,0 +1,92 @@
+"""
+   Execution:  python nonrecursive_depth_first_paths.py G s
+
+   Data files:   https://algs4.cs.princeton.edu/41graph/tinyCG.txt
+                 https://algs4.cs.princeton.edu/41graph/tinyG.txt
+                 https://algs4.cs.princeton.edu/41graph/mediumG.txt
+                 https://algs4.cs.princeton.edu/41graph/largeG.txt
+
+   Run nonrecurisve depth-first search on an undirected graph.
+   Runs in O(E + V) time using O(V) extra space.
+
+   %  python graph.py tinyCG.txt
+   6 8
+   0: 2 1 5
+   1: 0 2
+   2: 0 1 3 4
+   3: 5 4 2
+   4: 3 2
+   5: 3 0
+
+   % python nonrecursive_depth_first_paths.py tinyCG.txt 0
+   0 to 0:  0
+   0 to 1:  0-2-1
+   0 to 2:  0-2
+   0 to 3:  0-2-3
+   0 to 4:  0-2-3-4
+   0 to 5:  0-2-3-5
+
+"""
+from algs4.stack import Stack
+from algs4.graph import Graph
+
+
+class NRDepthFirstPaths:
+
+    def __init__(self, G, s):
+        self.marked = [False for _ in range(G.V)]
+        self.edge_to = [0 for _ in range(G.V)]
+        self.s = s
+
+        stack = Stack()
+        stack.push(s)
+
+        while not stack.is_empty():
+            v = stack.peek()
+            stack.pop()
+
+            if not self.marked[v]:
+                self.marked[v] = True
+
+            for node in G.adj[v]:
+                if not self.marked[node]:
+                    self.edge_to[node] = v
+                    stack.push(node)
+
+    def has_path_to(self, v):
+        return self.marked[v]
+
+    def path_to(self, v):
+        if not self.has_path_to(v):
+            return
+        path = Stack()
+        x = v
+        while x != self.s:
+            path.push(x)
+            x = self.edge_to[x]
+        path.push(s)
+        return path
+
+
+if __name__ == '__main__':
+    import sys
+    f = open(sys.argv[1])
+    s = int(sys.argv[2])
+    V = int(f.readline())
+    E = int(f.readline())
+    g = Graph(V)
+    for i in range(E):
+        v, w = f.readline().split()
+        g.add_edge(v, w)
+    dfs = NRDepthFirstPaths(g, s)
+    for v in range(g.V):
+        if dfs.has_path_to(v):
+            print("%d to %d: " % (s, v), end='')
+            for x in dfs.path_to(v):
+                if x == s:
+                    print(x, end='')
+                else:
+                    print('-%s' % x, end='')
+            print()
+        else:
+            print("%d and %d: not connected" % (s, v))
diff --git a/algs4/nonrecursive_depth_first_search.py b/algs4/nonrecursive_depth_first_search.py
new file mode 100644
index 0000000..6ee8dcb
--- /dev/null
+++ b/algs4/nonrecursive_depth_first_search.py
@@ -0,0 +1,62 @@
+"""
+  Execution:    python nonrecursive_depth_first_search.py filename.txt s
+  Data files:   https: // algs4.cs.princeton.edu / 41graph / tinyG.txt
+                https: // algs4.cs.princeton.edu / 41graph / mediumG.txt
+
+  Run nonrecurisve depth-first search on an undirected graph.
+  Runs in O(E + V) time using O(V) extra space.
+
+ % python nonrecursive_depth_first_search.py tinyG.txt 0
+  0 1 2 3 4 5 6
+  NOT connected
+
+ % python nonrecursive_depth_first_search.py tinyG.txt 9
+  9 10 11 12
+  NOT connected
+
+ """
+
+from algs4.graph import Graph
+from algs4.stack import Stack
+
+
+class NRDepthFirstSearch:
+
+    def __init__(self, G, s):
+        self.marked = [False for _ in range(G.V)]
+        self.count = 0
+
+        stack = Stack()
+        stack.push(s)
+
+        while not stack.is_empty():
+            v = stack.peek()
+            stack.pop()
+
+            if not self.marked[v]:
+                self.marked[v] = True
+                self.count += 1
+
+            for node in G.adj[v]:
+                if not self.marked[node]:
+                    stack.push(node)
+
+
+if __name__ == '__main__':
+    import sys
+    f = open(sys.argv[1])
+    s = int(sys.argv[2])
+    V = int(f.readline())
+    E = int(f.readline())
+    g = Graph(V)
+    for i in range(E):
+        v, w = f.readline().split()
+        g.add_edge(v, w)
+    search = NRDepthFirstSearch(g, s)
+    for v in range(g.V):
+        if search.marked[v]:
+            print(str(v) + " ")
+    if search.count == g.V:
+        print("connected")
+    else:
+        print("not connected")
diff --git a/algs4/stack.py b/algs4/stack.py
index 6198da8..d40871d 100644
--- a/algs4/stack.py
+++ b/algs4/stack.py
@@ -43,6 +43,11 @@ def pop(self):
             self.n -= 1
             return item
 
+    def peek(self):
+        if self.is_empty():
+            raise ValueError("Stack underflow")
+        return self.first.item
+
 
 if __name__ == '__main__':
     import sys