LeetCode:377. Combination Sum IV

LeetCode:377. Combination Sum IV

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Given an integer array with all positive numbers and no duplicates, find the number of possible combinations that add up to a positive integer target.

Example:

nums = [1, 2, 3]
target = 4

The possible combination ways are:
(1, 1, 1, 1)
(1, 1, 2)
(1, 2, 1)
(1, 3)
(2, 1, 1)
(2, 2)
(3, 1)

Note that different sequences are counted as different combinations.

Therefore the output is 7.

题目的意思就是给你一个正整数数组,里面的元素不重复(未排序)。再给你一个目标数值,要你用数组里面的数组合起来的和是目标数值,求出有多少种可能。

这道题目有点像那个爬梯子的题目。同时也可以看看这篇文章ClimbingStairs

两道题目有异曲同工之妙,我们来思考一下这道题目。如果我要求和为4的所有可能性,首先要求(4-1),(4-2),(4-3)的可能性,它们的和就是4的可能性。来看一下Java代码实现:

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public class Solution {
public int combinationSum4(int[] nums, int target) {
Arrays.sort(nums);
int[] res = new int[target+1];
for (int i = 1 ; i < target+1;i++) {
for(int num : nums) {
if(i == num) {
res[i]+=1;
} else if(i > num) {
res[i]+= res[i-num];
} else {
break;
}
}
}
return res[target];
}
}

THE END

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