LeetCode

LeetCode 206 note: three pointers prev / cur / nxt; save the next node, then point cur.next back at prev

LeetCode 205 note: characters must map one to one; it holds when the distinct characters and distinct pairs all have the same count

LeetCode 203 note: the head may be deleted too, so a dummy head turns every deletion into deleting cur.next; do not advance after deleting

LeetCode 202 note: repeatedly take the sum of squared digits, which reaches 1 or enters a cycle; detect it with a set of seen numbers or fast and slow pointers

LeetCode 171 note: scan left to right with res = res × 26 + letter value (A = 1)

LeetCode 169 note: Boyer-Moore voting, where different values cancel in pairs and the survivor is the one appearing more than half the time

LeetCode 168 note: Excel columns are base 26 without a zero digit; take (n - 1) % 26 as the letter, then n = (n - 1) // 26

LeetCode 160 note: each pointer walks its own list then the other’s; both cover lenA + lenB steps and reach the intersection, or None, together

LeetCode 145 note: iterative post-order pushes a node back with a visited flag on its first pop, then the right and left children; record it on the second pop

LeetCode 141 note: Floyd’s cycle detection, fast two steps and slow one; with a cycle they must meet, each advancing from its own position

LeetCode 136 note: x ^ x = 0 and x ^ 0 = x, so XOR everything; pairs cancel and the single number remains

LeetCode 125 note: two pointers skip non-alphanumeric characters and compare lowercase letters, with digits included