reduce time complexity O(n²) to O(n)

the solution can be optimized by calculating the height in the same recursion  rather than calling a height() function separately.
Merge branch 'patch-1' of https://github.com/FreeJoker/leetcode-1 into FreeJoker-patch
update the problem "Balanced Binary Tree"
Merge branch 'FreeJoker-patch'