打家劫舍问题（根据递推公式来初始化
偷i和不偷i两种情况
如果连成环，考虑首元素就不考虑尾元素，考虑尾元素就不考虑首元素
线性问题代码
class Solution {public int rob(int[] nums) {int len=nums.length;int[] dp=new int[len];//记录最大可偷窃金额dp[0]=nums[0];dp[1]=Math.max(dp[0],nums[1]);for(int i=2;i<len;i++){dp[i]=Math.max(dp[i-1],dp[i-2]+nums[i]);}return dp[len-1];}
}环形问题，start相当于遍历nums数组的指针，i相当于遍历dp[] 数组的指针
class Solution {public int rob(int[] nums) {int len = nums.length;if (len == 1) return nums[0];if (len == 2) return Math.max(nums[0], nums[1]);int r1 = robHelper(nums, 0, len - 2);int r2 = robHelper(nums, 1, len - 1);return Math.max(r1, r2);}public int robHelper(int[] nums, int start, int end) {int[] dp = new int[end - start + 1];//数组长度！dp[0] = nums[start];dp[1] = Math.max(nums[start], nums[start + 1]);for (int i = 2; i <= end - start; i++) {dp[i] = Math.max(dp[i - 1], dp[i - 2] + nums[i + start]);}return dp[end - start];}
}
二叉树问题（用后序遍历递推，res[]记录偷与不偷的值
class Solution {public int rob(TreeNode root) {int[] result=robHelper(root);return Math.max(result[0],result[1]);}public int[] robHelper(TreeNode root){int[] res=new int[2];//res[0-1] 记录偷与不偷的值if(root==null){return res;}int[] left=robHelper(root.left);int[] right=robHelper(root.right);int value1=root.val+left[0]+right[0];//偷当前结点int value2=Math.max(left[0],left[1])+Math.max(right[0],right[1]);//不偷当前结点res[1]=value1;//偷，注意要对应res[0]=value2;return res;}
}
)
)
