刷题打卡 －pxu

295. Find Median from Data Stream － resolved with max and min heap. make sure the size of those two are balanced.

305 Number of Islands II - resolved it with union find. tricky:
int rootOfCurr = find(roots, curr);
                if(rootOfCurr != root){
                    roots[root] = rootOfCurr;
                    count--;
                   root = rootOfCurr;
                }

355. Design Twitter  - I worked this all by myself and AC, yeah. after reviewing the discussion, I abstract User class with will implement follow and unfollow with it. more like OOD question now.

just refactored the twitter codes and feel much better now. so, it's not just making it work, it should ood orientated

207. Course Schedule  - using dfs and go throught each nodes

348. Design Tic-Tac-Toe . First, I was using OOD to keep all the data and was able to AC. After reviewing the discussion, one of the them is brilliant. it only checked the sum so far, e.g.    int rows[], cols[], diagonals, antiDiagonals;     int size;     /** Initialize your data structure here. */     public TicTacToe(int n) {         size = n;         rows = new int[n];         cols = new int[n];     }

#186 Reverse Words in a String II
forgot to shift left++ and right--
remember to handle the edge case(i.e in the end)

#325Maximum Size Subarray Sum Equals k added map.put(0, -1) to make it easy to calculate distance try to finish everything in one loop

#Longest Palindromic Substring - Forgot to write the  else case if(s.charAt(left) == s.charAt(right)){}else{return;}. remembered to shift left to left and right to right side

238. Product of Array Except Self - using leftside and rightside to handle multiplication before it self.

for(int i = 1; i < nums.length; i++){
            res[i] = leftSides * nums[i-1];
            leftSides = res[i];
        }
        
        for(int i = nums.length -2; i >=0; i--){
            res[i] *= rightSides * nums[i+1];
            rightSides = rightSides * nums[i+1];
        }