Leetcode 精选之搜索（完全平方数）

题目描述

给定正整数 n，找到若干个完全平方数（比如 1, 4, 9, 16, ...）使得它们的和等于 n。你需要让组成和的完全平方数的个数最少。

示例 1:

输入: n = 12
输出: 3
解释: 12 = 4 + 4 + 4.
示例 2:

输入: n = 13
输出: 2
解释: 13 = 4 + 9.

题目链接：力扣

解题思路

class Solution {
    public int numSquares(int n) {
        List<Integer> squares = generateSquares(n);
        Queue<Integer> queue = new LinkedList<>();
        boolean[] marked = new boolean[n + 1];
        queue.add(n);
        marked[n] = true;
        int level = 0;
        while (!queue.isEmpty()) {
            int size = queue.size();
            level++;
            while (size-- > 0) {
                int cur = queue.poll();
                for (int s : squares) {
                    int next = cur - s;
                    if (next < 0) {
                        break;
                    }
                    if (next == 0) {
                        return level;
                    }
                    if (marked[next]) {
                        continue;
                    }
                    marked[next] = true;
                    queue.add(next);
                }
            }
        }
        return n;
    }

    /**
    * 生成小于 n 的平方数序列
    * @return 1,4,9,...
    */
    private List<Integer> generateSquares(int n) {
        List<Integer> squares = new ArrayList<>();
        int square = 1;
        int diff = 3;
        while (square <= n) {
            squares.add(square);
            square += diff;
            diff += 2;
        }
        return squares;
    }
}

测试结果

image.png