LeetCode - Triangle

最简单的递归写法，可以pass Judge Small, TLE Judge Large

public class Solution {
    public int minimumTotal(ArrayList<ArrayList<Integer>> triangle) {
        // Start typing your Java solution below
        // DO NOT write main() function
        return recursion(triangle, 0, 0);
    }
    
    public int recursion(ArrayList<ArrayList<Integer>> triangle, int row, int col) {
        if (row >= triangle.size())
            return 0;
        ArrayList<Integer> line = triangle.get(row);
        int value = line.get(col);
        return value + Math.min(recursion(triangle, row+1, col), recursion(triangle, row+1, col+1));
    }
}

改为非递归试试先

public class Solution {
    public int minimumTotal(ArrayList<ArrayList<Integer>> triangle) {
        // Start typing your Java solution below
        // DO NOT write main() function
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        
        for (int i=triangle.size()-1; i>=0; i--) {
            ArrayList<Integer> row = new ArrayList<Integer>();
            ArrayList<Integer> triLine = triangle.get(i);
            for (int j=0; j<triLine.size(); j++) {
                int value = triLine.get(j);
                if (result.size() > 0) {
                    ArrayList<Integer> riRow = result.get(result.size()-1);
                    value += Math.min(riRow.get(j), riRow.get(j+1));
                }
                row.add(value);
            }
            result.add(row);
        }
        return result.get(result.size()-1).get(0);
    }
    
    public int recursion(ArrayList<ArrayList<Integer>> triangle, int row, int col) {
        if (row >= triangle.size())
            return 0;
        ArrayList<Integer> line = triangle.get(row);
        int value = line.get(col);
        return value + Math.min(recursion(triangle, row+1, col), recursion(triangle, row+1, col+1));
    }
}

可以通过Judge Large,但是题目说Bonus point if you are able to do this using only O(n) extra space, where n is the total number of rows in the triangle.,想想这么该怎么弄。。。

public class Solution {
    public int minimumTotal(ArrayList<ArrayList<Integer>> triangle) {
        // Start typing your Java solution below
        // DO NOT write main() function
        Integer[] array = triangle.get(triangle.size()-1).toArray(new Integer[0]);
        for (int i=triangle.size()-2; i>=0; i--) {
            ArrayList<Integer> row = triangle.get(i);
            for (int j=0; j<row.size(); j++) {
                array[j] = row.get(j) + Math.min(array[j], array[j+1]);
            }
        }
        return array[0];
    }
}

其实，如果可以复用原来的triangle的话，都不需要额外的存储空间