stack to vector, init from stack size and --i to change the vector index value

Author: Adam Lin

735_asteroidCollision.cpp

@@ -0,0 +1,66 @@
+#include<iostream>
+#include<vector>
+#include<stack>
+#include<algorithm>
+
+using namespace std;
+
+class Solution {
+public:
+    vector<int> asteroidCollision(vector<int>& asteroids) {
+        stack<int> stk;
+
+        for(auto var : asteroids){
+            bool explode = false;
+            // A collision can occur only when a left-moving asteroid (var < 0) meets a right-moving asteroid (stk.top() > 0).
+            while(!stk.empty() && var < 0 && stk.top() > 0){
+                if(stk.top() < -var){
+                    stk.pop();
+                    continue;
+                } else if(stk.top() == -var){
+                    stk.pop();
+                    explode = true;
+                    break;
+                } else {
+                    explode = true;
+                    break;
+                }
+            }
+            if(!explode){
+                stk.push(var);
+            }
+        }
+        
+        vector<int> res(stk.size());
+        for(int i = stk.size()-1; i >= 0; --i){
+            res[i] = stk.top();
+            stk.pop();
+        }
+        return res;
+    }
+};
+
+
+int main(){
+    Solution sol;
+    vector<int> asteroids = {-2,-1,1,2};
+    sol.asteroidCollision(asteroids);
+    return 0;
+}
+
+/*
+stk.top() < -var:
+Explanation: The right-moving asteroid on the stack is smaller than the left-moving asteroid (|stk.top()| < |var|).
+Action: The right-moving asteroid is destroyed (stk.pop()).
+continue;: Loop continues to check for further possible collisions with other right-moving asteroids on the stack.
+stk.top() == -var:
+Explanation: Both asteroids are of equal size (|stk.top()| == |var|).
+Action: Both asteroids are destroyed.
+stk.pop();: Remove the right-moving asteroid from the stack.
+destroyed = true;: Mark the current left-moving asteroid as destroyed.
+break;: Exit the while loop since the current asteroid is destroyed.
+else:
+Explanation: The right-moving asteroid on the stack is larger than the left-moving asteroid (|stk.top()| > |var|).
+Action: The left-moving asteroid is destroyed (destroyed = true;).
+break;: Exit the while loop since the current asteroid is destroyed.
+*/