05 August 2008
找到所有的paths，不必非是root开始到leaf结束，只要向下的路径就行
preSum
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/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode() {}
* TreeNode(int val) { this.val = val; }
* TreeNode(int val, TreeNode left, TreeNode right) {
* this.val = val;
* this.left = left;
* this.right = right;
* }
* }
*/
class Solution {
public int pathSum(TreeNode root, int sum) {
HashMap<Integer, Integer> preSum = new HashMap();
preSum.put(0,1);
return helper(root, 0, sum, preSum);
}
public int helper(TreeNode root, int currSum, int target, HashMap<Integer, Integer> preSum) {
if (root == null) {
return 0;
}
currSum += root.val;
int res = preSum.getOrDefault(currSum - target, 0);
preSum.put(currSum, preSum.getOrDefault(currSum, 0) + 1);
res += helper(root.left, currSum, target, preSum) + helper(root.right, currSum, target, preSum);
preSum.put(currSum, preSum.get(currSum) - 1);
return res;
}
}