Problem 473: Matchsticks to Square
Problem 473: Matchsticks to Square
https://leetcode.com/problems/matchsticks-to-square/#/description
思路
之前的题目是平分成两份,这里升级了以后,相当于是平分成四份。
很难用 dp 去解决,这里用了最简单的 dfs 的思路。
在 dfs 之前,加入了 sort array 的过程,这样可以优化 dfs 的过程
整个 dfs 的过程还是不好理解的。每次在判断的时候,count 加 1,进入下一层去判断;儿判断之后,再把 nums 的元素减去
public class Solution {
public boolean makesquare(int[] nums) {
if (nums == null || nums.length < 4) return false;
int sum = 0;
for (int num : nums) {
sum += num;
}
if (sum % 4 != 0) return false;
Arrays.sort(nums);
reverse(nums);
return dfs(nums, new int[4], 0, sum / 4);
}
private boolean dfs(int[] nums, int[] sums, int count, int target) {
if (count == nums.length) {
if (sums[0] == target && sums[1] == target && sums[2] == target) return true;
}
for (int i = 0; i < 4; i++) {
if (sums[i] + nums[count] > target) continue;
sums[i] += nums[count];
if (dfs(nums, sums, count + 1, target)) return true;
sums[i] -= nums[count];
}
return false;
}
private void reverse(int[] nums) {
int len = nums.length;
for (int i = 0; i < len / 2; i++) {
int tmp = nums[i];
nums[i] = nums[len - 1 - i];
nums[len - 1 - i] = tmp;
}
}
}
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