[leetcode]120. Triangle(Java)

https://leetcode.com/problems/triangle/#/description

Given a triangle, find the minimum path sum from top to bottom. Each step you may move to adjacent numbers on the row below.

For example, given the following triangle

[
[2],
[3,4],
[6,5,7],
[4,1,8,3]
]

The minimum path sum from top to bottom is 

11

 (i.e., 2 + 3 + 5 + 1 =
11).

Note:

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.

package go.jacob.day714;

import java.util.List;

public class Demo1 {
/*
* 从下往上遍历，不需要开辟额外的空间
*/
public int minimumTotal(List<List<Integer>> triangle) {
int rows=triangle.size();
for(int i=rows-2;i>=0;i--){
for(int j=0;j<=i;j++){
int min=Math.min(triangle.get(i+1).get(j), triangle.get(i+1).get(j+1));
triangle.get(i).set(j, triangle.get(i).get(j)+min);

}
}
return triangle.get(0).get(0);
}

/*
* Solution by me
* 需要额外空间
*/
public int minimumTotal_1(List<List<Integer>> triangle) {
int rows = triangle.size();
int[][] res = new int[rows][rows];
res[0][0] = triangle.get(0).get(0);
for (int i = 1; i < rows; i++) {
for (int j = 0; j <= i; j++) {
int num = triangle.get(i).get(j);
int min = Math.min(j - 1 >= 0 ? res[i-1][j-1] : res[i-1][j],
j < i ? res[i-1][j] : res[i-1][j-1])
;
res[i][j] = num + min;
}
}
int min = res[rows - 1][0];
for (int i = 1; i < rows; i++)
if (min > res[rows - 1][i])
min = res[rows - 1][i];
return min;
}
}


4000