刷题打卡 －pxu

763. Partition Labels
make sure max(max, hash[i]) to get the updated end;
check current i == end

762. Prime Number of Set Bits in Binary Representation
try to get the count of 1s in the binary;
1 is not prime and 2 is the primary, make sure handle that in the isPrime method
for(int i = 2; i*i <= num;i++){

535. Encode and Decode TinyURL
used hashmap and Random class

380. Insert Delete GetRandom O(1)
public boolean remove(int val) {
        // write your code here
        if(!posMap.containsKey(val)){
            return false;
        }
        
        Integer lastVal = list.get(list.size()-1);
        int posLast = posMap.get(lastVal);
        int posOfVal = posMap.get(val);
        
        if(posLast != posOfVal){
            posMap.put(lastVal, posOfVal);
            list.set(posOfVal,lastVal);
        }
        
        list.remove(list.size()-1);
        posMap.remove(val);
        return true;
        
    }

74. Search a 2D Matrix - AC,easy

235. Lowest Common Ancestor of a Binary Search Tree
make sure return when root = p or q

236. Lowest Common Ancestor of a Binary Tree
TreeNode left = lowestCommonAncestor(root.left, p, q);
TreeNode right = lowestCommonAncestor(root.right, p, q);

if(left != null && right != null){
    return root;
}

234. Palindrome Linked List.txt
the stack should push data < len/2; skip value check on len/2 and odd len

92. Reverse Linked List II
find preM, mNode and nNode postion and keep on move mNode after nNode, and point mNode to preM.next, util mNode == nNode

204. Count Primes
isPrime[]
其算法是，给出要筛数值的范围, 找出以内的素数. 先用2去筛，即把2留下，把2的倍数剔除掉；再用下一个素数，也就是3筛，把3留下，把3的倍数剔除掉；接下去用下一个素数5筛，把5留下，把5的倍数剔除掉；不断重复下去。

119. Pascal's Triangle II.txt
append 1 on every look, update value from right to left;

for(int i = 0; i <= rowIndex; i++){
            res.add(1);
            
            for(int j = i - 1; j > 0; j--){
                res.set(j, res.get(j-1) + res.get(j));
            }
        }