路径总和 III——力扣刷题2021-09-28

题目链接

思路

二叉树的深度遍历,穷举所有可能

代码

class Solution {
          
   
    public int pathSum(TreeNode root, int targetSum) {
          
   
        if(root == null)
            return 0;
        int path += path(root,targetSum);
        path += pathSum(root.left,targetSum);
        path += pathSum(root.right,targetSum);
        return path;
    }

    public int path(TreeNode root,int targetSum){
          
   
        int res = 0;
        if(root == null)
            return 0;
        int val = targetSum - root.val;
        if(val == 0)
            res++;
        res += path(root.left,val);
        res += path(root.right,val);
        return res;
    }
}

优化

    思路 前缀和 代码 class Solution { public int pathSum(TreeNode root, int targetSum) { HashMap<Integer, Integer> prefix = new HashMap<>(); prefix.put(0, 1); return dfs(root, prefix, 0, targetSum); } public int dfs(TreeNode root, Map<Integer, Integer> prefix, int curr, int targetSum) { if (root == null) { return 0; } int ret = 0; curr += root.val; ret = prefix.getOrDefault(curr - targetSum, 0); prefix.put(curr, prefix.getOrDefault(curr, 0) + 1); ret += dfs(root.left, prefix, curr, targetSum); ret += dfs(root.right, prefix, curr, targetSum); prefix.put(curr, prefix.getOrDefault(curr, 0) - 1); return ret; } }
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