osigmao的专栏

先把digit-letter建个Given a digit string, return all possible letter combinations that the number could represent.

public class Solution { public List> combinationSum(int[] candidates, int target) { if (candidates == null || candidates.length == 0) { return null; } Arrays.so

枚举的题，一般用DFS+Backtracking做，此题会超时，那么要辅助memoization, 保存之前得到的结果.怎么mem？1，借鉴word break I, 保存能否该index之后能否break： boolean[s.length() + 1]2,  保存详细的分法结果： HashMap>public class Solution { List res =

Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.For example,If n = 4 and k = 2, a solution is:[ [2,4], [3,4], [2,3], [1,2], [1,3], [1,4],]

DFS + BacktrackingII是考虑原set中有重复元素的情况Subset ISubset I