107. Binary Tree Level Order Traversal II

• use element as index to record whether visit

117. Populating Next Right Pointers in Each Node II

• child connected and be used for next level

646 Maximum Length of Pair Chain

• DP: pairs[i][0] > pairs[j][1]? dp[j] + 1 : dp[j]
• greedy: for end point z, if all intervals where end point < z, there is no possible to get a longer chain based on previous intervals.

661 Image Smoother

129. Sum Root to Leaf Numbers