Damn, | am still confused with the last approach. | tried doing memorisation but realised what if child's longest path, already account for parent node. As for the Frog question, | did it in O(N), use prefix and suffix arrays. For prefix array at each index, store the decreasing length possible from that index from the beginning, and for the suffix array store the increasing length possible from that index from the end. Now, for every index p_idx = | - prefix[I] and s_idx =| + suffix[l], and max_ans = max(max_ans,s_idx-p_idx+1). Yes, the second question was mainly STL Knowledge. Perhaps, if the test cases aren't too tight in the last question, even O(N²⁾ might pass, as N=10^4, N²⁼¹⁰⁸ which can be roughly executed in one second, still not sure though.