331, use stack, replace (number, #, #) with # while scanning the preorder

340, sliding window

339

146

91, dp

72, dp

70, f(n) = f(n - 1) + f(n - 2), fibonacci number

67

66, check current digit is 9 or not

65, easier ask for forgiveness than permission

64, initialize 1st row and 1st column, then dp

63, initialize first row and first column, then dp

62, combination, m ↓ steps, n → steps

61, go to end, go to k, switch

60, get candidate[index] by curr_k / (n - 1)!, curr_k / (n - 2)!, ... curr_k / 0!